Question

10^2=x•(x+3x)

Use quadratic formula to solve

Answer 1

100 = 4x^2

25 = x^2
x = 5 or -5

but if you really want to use formula

x^2 + 0 x - 25 = 0

x = [ -0 +/- sqrt (0 + 100) ] / 2

x = +/- 10/2

x = +/- 5