Is it possible to find plaintext from ciphertext if (n) and (a) are known for RSA?
For small numbers the easiest ways to break RSA are:


Brute force the private (decryption) exponent. I.e. find the smallest value $b$ such that $(x^a)^b \mod n = x$ for all $x$.
Factorize the modulus (trial division is simplest and works fine for small numbers), then use the factors to generate the private exponent.


Assuming you are supposed to use the maths, 2. is probably expected.