X^2 +x-12 = 0 if a is a solution of the equation above and a> 0, what is the value of a?

X² + x - 12 factors into (x + 4) and (x - 3)
So if you set each factor equal to zero you get the following solutions
(x + 4) = 0   ⇒ x = -4  which cannot be "a" since a > 0 and -4 is not > 0
(x - 3) = 0   ⇒ x = 3 this is the value of "a"

a = 3

The answer for you question is b