To solve for the inverse of the function, one way to go about it is to switch or reverse the variables for both x and y. Then solve for y or f(X) again.
F(X) = 3x+5/7
Y = 3x+5/7
X = 3y+5/7
7x = 3y+5
7x-5 = 3y+5-5
7x-5 = 3y
7x-5/3 = 3y/3
7x-5/3 = y
Y = 7x-5/3
F(x) = 7x-5/3.