The average, or mean, $T$, of three exam grades, $r, s$, and $y$, is given by the following formula.
\[
T=\frac{r+s+y}{3}
\]
(a) Solve the formula for $y$.
(b) Use the formula in part (a) to solve this problem. On your first two exams, your grades are $84 \%$ and $89 \%$ : $r=84$ and $s=89$. What must you get on the third exam to have an average of $89 \%$ ?
(a) The formula is $y=$

Step 1 ：(a) To solve the formula for $y$, we start with the given formula $T=\frac{r+s+y}{3}$. Multiply both sides by 3 to get $3T=r+s+y$. Then, subtract $r$ and $s$ from both sides to isolate $y$. So, $y=3T-r-s$.

Step 2 ：(b) To find out what you must get on the third exam to have an average of $89\%$, we substitute $T=89$, $r=84$, and $s=89$ into the formula we just derived. So, $y=3*89-84-89$.

Step 3 ：Calculate the right side of the equation to get $y=267-84-89$.

Step 4 ：Further simplify to get $y=94$.

Step 5 ：So, you must get a $94\%$ on the third exam to have an average of $89\%$. The final answer is \(\boxed{94}\).