Let the test scores be a, b and c.
Then the average is (a + b + c) / 3 which you know is 84.
You know that the first score is 67 (so a = 67) and the second score
is 7 less than the third (so b = c - 7).
Let's re-write that first expression and replace a and b with what they equal:
(a + b + c) / 3 = 84
(67 + (c - 7) + c) / 3 = 84
Do you think you can solve this last equation and find the third score?
Hope this helps,
Stephen La Rocque.