It all depends on how much the final exam is weighted.

The final grade is a weighted average of the midterms, homework, quizzes, tests, maybe class room attendance or participation, plus the final.

Usually the syllabus tells you how they're weighted. It's a weighted average.

If the final exam counts 40% of the final grade, then 60% counts as 89

.6(89) plus .4(x) = 90 with x what you need for the final exam grade to get a 90. Solve the equation for x

53.5 + .4x = 90

.4 x = 90-53.5 = 36.5

x = 36.5/.4 = 365/4 = 91.25

You need at least a 91 1/4 on the final exam if it's weighted 40%

Then it also depends on how they round fractions. If you get 89.5, some instructors or most, round up,

then all you need is a weighted average of 89.5

then .4x = 89.5-53.5 = 36

x = 360/4 = 90. Then "just" a 90 is all you need.