(87 + 92 + 84 + 2x)/5 = 90
(263 + 2x)/5 = 90
Now solve for x.
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.