If f(a) = k, then which of the following must be true? f(k) = a f^-1(k)=a f^-1(a)=k

Rick is driving to his uncles house in greenville,which is 120 miles from ricks town .after covering x miles rick she's a sign stating that greensville is 20 miles away. which equation when solved will give the value of x. a: x+120 = 20 b: x x120 =20 c: x +20 equals 120 d: x x20= 120