You don't need to use the quadratic equation for that one, you use the quadratic equation when you have something in the form ax^2+bx+c=0. Solving that equation is just like any old equation from pre-algebra.
Original Equation
8x^2=72
Divide both sides by 8
x^2=9
Square root of both sides
x= + or - 3
Early I said you don't have to use the quadratic equation to solve, however you can use it if you want to.
Simply rewrite the equation so it's equal to 0.
8x^2-72=0
What you have in effect is
8x^2+0x-72=0
Your a will equal 8, b will equal 0 and c will equal -72. From there you simply plug into the formula.