find a formula for f^-1(x)
f(x)=x+15/x-15
Just on some hunch, I will assume you meant
f(x)=(x+15)/(x-15) or
y = )=(x+15)/(x-15
step 1, interchange the x and y
x = (y+15)/(y-15)
step 2, solve this new equation for y
xy-15x = y+15
xy - y = 15x + 15
y = 15(x+1)/(x-1)
f^-1 (x) = 15(x+1)/(x-1)