Set the test score equal to the curve's formula. The formula will have an exponent in place of the raw score. The exponent will commonly be designated by an X. If your test score is 91, and the curve's formula is 84+8/9(X-78), then 91=84+8/9(X-78).
Get X alone on one side of the equation. If 91=84+8/9(X-78), then first subtract 84 from each side, which will give you 7=8/9(X-78). Next, divide each side by 8/9 which will give you 7.875=X-78. Then, add 78 to each side, which will give you 85.875=X.
X is your raw score. A raw score is your score before it was curved. Find the difference between your raw score and your test score. If your raw score is 85.875 and your test score is 91, then the curve of your test score is 91-85.875 which equals 5.125.