If the fourth test is counted twice as much, simply recopy your previous equation but multiply the x by 2, and divide the sum by 5:

(87 + 92 + 84 + 2x)/5 = 90

(263 + 2x)/5 = 90

Now solve for x.

01

EDIT: This is assuming you mean that the fourth test is counted twice as much as any of the other 3 tests, as opposed to counting twice as much as the combined score of the three tests.

EDIT #2: I misread your equation. You forgot the parentheses earlier.