(a) just substitute x=800 into the equation for T
(b) set T=9 and solve the quadratic to find possible values for x. If any value lies outside the range 500...800 then discard it for this exercise.
You have memorised a general formula for solving a quadratic, use it here. π
Here's a refresher: http://www.purplemath.com/modules/quadform.htmβ