Answer
$-90^\circ+n\times 360^\circ$
Work Step by Step
In order to find the coterminal angles of a quadrantal angle, we have to add or subtract $360^\circ$.
The expression that generates all coterminal angles is:
$-90^\circ+n\times 360^\circ$, as $n$ can be any integer, we can add or subtract any multiple of $360^\circ$.