Factorising this gives you:
(x+4)(x-1) = 0
You get this because +4 and -1 add together to make 3 (the number in front of the x) and multiply to make -4 (the number on its own).
Now you know that either:
x + 4 = 0
or
x - 1 = 0
(Because when two numbers multiplyed together make 0, one of them must be zero).
This leads to the answer:
x = -4 or x = 1
