You have one equation and two unknown quantities. You can't solve it. More information is needed.
Add parentheses to clarify meaning:
800/(p + w) = 720/(p - w)
Define your terms explicitly.
p = speed of plan in still air
w = speed of wind
Motivate your equation(s). Did you pull it(them) out of the air? Where did it(they) come from?
p + w = 800 mph
p - w = 720 mph
Ah, now that's two equations with two unknown quantities. We have a chance to solve it.
Note: Your problem statement mentions neither time nor distance. If you are trying to cram this problem into something you memorized for some other problem, you'll not make it through. Generally, memorize concepts, not formulas. Some formulas are of sufficient importance - memorize those few.