Okay, we can do this! First, we need to add up all of the scores, even the ones we don't know yet:
92 + 87 + 89 + 94 + X
362 + X
Now then: we need to find the average grade, and that means totaling them up (we did that already!) and dividing by the number of terms (5). Like this:
(362 + x) / 5 ≥ 90
Multiply both sides by 5 and we get:
362 + X ≥ 450
X ≥ 88See? We're done already!