Because the highest power of X is 2, this is a Quadratic Equation
When factorised, these look like
(x + a)(x + b) where a & b are constants
To factorise this, we need to find 2 numbers which add together to make the coefficient (number in front of) X and multiply to make the number at the end.
So we need two numbers which add together to make +1 and multiply to make -12 +4 & -3
a & b in the form above are replace by these to numbers, so the answer is
(x+4)(x-3)