This is a quadratic equation so there are two main methods you can use to solve it- factorising and completing the square.
My preferred method and the one I will demonstrate is factorisation.
The above equation will take the form:
(x + a)(x +b) = 0
Therefore if we multiply out the brackets we get:
x^2 + (a+b)x +ab = 0
This means that
(a+b) = 1 (the coefficient of x)
and
ab = 12
From trial and error we find the values for a and b which are
a= -3
b= 4
So x^2 + x -12= 0 can be written as (x-3)(x+4)= 0
When we multiply by 0 we get 0 therefore
x-3 = 0 or
x+4= 0
From rearranging the above equations we find the answer is x = 3 or x = -4