To find the measure of angle \( x \) that is supplementary to \( 15^\circ \), we can use the fact that supplementary angles add up to \( 180^\circ \).
We can set up the equation:
\[ x + 15^\circ = 180^\circ \]
To solve for \( x \), we can subtract \( 15^\circ \) from \( 180^\circ \):
\[ x = 180^\circ - 15^\circ \]
\[ x = 165^\circ \]
Thus, the measure of angle \( x \) is \( 165^\circ \).
So, \( x = [165]° \).