Which function below is the inverse of f (x) = 3x+5/7

To get the inverse of a function you switch f(x) - (which is really just y) - with x in the problem and try to get f(x) - (y) - alone again.

For this equation, it will look like this
Original equation: y=3x+5/7
Switch x and y: x=3y+5/7
solve to get y alone:

x       =3y+5/7
  -5/7       -5/7
x-5/7=3y+0
x-5/7=3y
divide both sides by 3
x/3 -(5/7)/3 = 3y/3
x/3 -5/21 = y
switch the equation so y is on the left
y=x/3-5/21

y=x/3-5/21 would be the inverse :)

Bbbhkkk