Answer
Two positive coterminal angles:
$270^\circ+360^\circ=630^\circ$
$270^\circ+2\times360^\circ=990^\circ$
Two negative coterminal angles:
$270^\circ-360^\circ=-90^\circ$
$270^\circ-2\times360^\circ=-450^\circ$
Work Step by Step
In order to find the coterminal angles of a quadrantal angle, we have to add or subtract $360^\circ$.
Two positive coterminal angles:
$270^\circ+360^\circ=630^\circ$
$270^\circ+2\times360^\circ=990^\circ$
Two negative coterminal angles:
$270^\circ-360^\circ=-90^\circ$
$270^\circ-2\times360^\circ=-450^\circ$