What is the inverse function of -5 (x-4) ^2+80

F(x)=-5(x-4)^2+80 
f(x)=-5(x²-8x+16)+80 
f(x)=-5x²+40x-80+80 
f(x)=-5x²+40x 

Replace f(x) to y, now switch the location of the variables x and y. Solve for new y. 

y=-5x²+40x 
x=-5y²+40y 
0=-5y²+40y-x 

Use quadratic formula to find the value of y. 

y=-40+-√(40^2 -4(-5)(-x)/2(-5)
f^-1(x)= (-40+-√1600-20x)/-10 <--- Not too sure about this one :/

Hope I helped :) 

Igot 108 hope that’s right !