Answer
At least 96.
Work Step by Step
First, let $x$ be the grade on the final examination.
Then from the asked problem we have the inequality:
$ \frac{86+88+x}{3} \geq 90$
Next, all we have to do is solve the inequality.
$ \frac{86+88+x}{3} \geq 90$
Multiply with $3$ on both sides.
$ \frac{86+88+x}{3}\times3 \geq 90\times3$
$86+88+x \geq 270$
$174+x \geq 270$
Subtract $174$ on both sides.
$174+x -174 \geq 270-174$
$ x \geq 96$
Which is our solution.
