The two red angle markers (the arcs) indicate that we have congruent angles. So this is an isosceles triangle making the two sides 24 and x^2+5x to be equal in length
Set the expressions equal to each other and solve for x
x^2+5x = 24
x^2+5x-24 = 0
(x-3)(x+8) = 0
x-3=0 or x+8=0
x = 3 or x = -8
So x is either 3 or -8
Plugging either x value into x^2+5x will have it turn into 24