Given: x²+x=12 ··· Eq.1
First, subtract 12 from both sides to get the standard form of quadratic equation:
x²+x-12=0 Factor the left side of equation, getting
(x-3)(x+4)=0 Thus, we have either x-3=0 or x+4=0. That is: x=3 or x=-4
CK: Plug x=3 or x=-4 into the left side of Eq.1:
3²+3=12 or (-4)²+(-4)=12 In either case LHS=RHS. CKD.
The answer: x=3 or x=-4