What does it mean by 'solve the equation'?

Question

3b^2 + 26b + 35 = 0

^ now what i want to do is use this 'x = - b +/- squareroot b^2-4ac over 2a

for this one
2z^2 +15z=8
how did they come up with -12, 12?

Answer 1

2z^2 +15z - 8 = 0
(2z -1)(z+8) = 0
2z = 1 --> z = 1/2
z = -8
-12 and 12 are incorrect answers
z = 1/2 and -8 are correct answers

Answer 2

3b^2 + 26b + 35 = 0
so
(3b+5)(b+7) = 0
so
b= -5/3 or -7

2z^2 +15z=8
so
(2z-1)(z+8)=0
so z=1/2 or -8
.