i think you mean solve the quadratic equation, you cant "perform" an equation. first just to clarify, are you sure it wasn't 9x^2 - 25 = 0?

if not, the equation has no real solutions, the solutions will be complex numbers. so here goes.

9x^2 + 25 = 0

=> 9x^2 = -25

=> x^2 = -25/9

=> x = +/- sqrt(-25/9)

=> x = +/- sqrt(25/9)i

=> x = +/- 5/3 i