Is this method of string encrypting "uncrackable"? I was thinking of unique methods of encrypting strings, and I thought of this.

Given a string, for example "Hello, World!" we can convert it into a char array using a computer program, and then convert each char into the corresponding number from the ASCII charcode table. (For example, H is 72, e is 101 etc.) then multiply each number by a random "key", which is just an integer (which will be the same for each number), in this case lets say it is 3042 (So 72 is now 219024, 101 is now 307242).

Then, remove from each number a value, which will be our second key (In this case, lets say it's 287), So 219024 becomes 218737 etc.

Now you see, if 2nd key wasn't removed from the result of the number * key1, then we could simply find the common factors of all the numbers, and grab the key.

However since a second key is removed, is this method of encryption practically uncrackable without knowing at least the 2nd key?
What you describe is a sightly convoluted version of a substitution cypher.  Every letter in the plaintext is replaced with an integer in the cyphertext.  If I have two 'e's, both will encode to the same value.

Unfortunately, in this case, the algorithm is far from uncrackable. There's a famous phrase, "Anyone can create an encryption algorithm so complicated that even they cannot crack it." This is one of those cases.  All substitution cyphers fall victim to statistical analysis.  If you have any information as to the statistical likelyhood of letters or words, you can rather rapidly deduce the keys.  This kind of cypher was considered unbreakable in the era of Caesar, but since the formalization of probability in the 1600's, this kind of encryption is considered "fully broken."

The fastest approach to break this algorithm is to gather enough characters to be confident that you have two sequential characters (such as an 'a' and a 'b'   or a 'g' and an 'h').  This can be guaranteed in 14 unique characters if you limit yourself to uppercase letters, or 129 characters if you have the entire 8-bit ASCII space at your disposal (the pigeonhole principle guarantees that if you have that many characters, at least one pair of them must be sequential).  Once you have enough characters, simply sort them and find the smallest gap.  The letters with the smallest gap between them are sequential.  Once you have those two, you can do simple algebra to get the two keys.

If you attempt to avoid this hole by only using half of the alphabet (acegikmoprtvxz, for instance), the attacker will see gibberish if they make this attack.  When they realize this, they can simply look at multiples of the key they generated until the messages start making sense.