How can I make a list like this?

Assume `a = [1, 2]` and I want to print the result `[4,5]`

I tried:

```
a = [1, 2]+[3, 3]
print(a) 

```

but the result shows: `[1, 2, 3, 3]`

I know that `[1, 2] + 3` is an error because list+integer is not possible.