You already have the equation. You just need to solve for x.
Like the problem says, your output (y) is 2/3 and the function is given. So 2/3 = -1/3x + 7. The first step has already been done: subtracting 7 from both sides to begin getting the x by itself. Because -7 is the same as -21/3, that’s why there’s a -19/3. Now all you have to do is get x by itself. To do that you have to multiply by the reciprocal of its coefficient: -1/3, which is -3. So multiply both sides by -3, to get -19 = x. And that’s your input.