Finding averages...?

Question

Suppose your grades on your first four exams were 84%, 94%, 65%, and 87%. What would be the lowest possible average that your last two exams could have so that your grade in the class, based on the average of the six exams, is at least 76%

I don't need the answer, just the formula so I know how to find the answer in the future.

Answer 1

(84 + 94 + 65 + 87 + x + x) / 6 = 76

solve for x