Answer
The cosine of that angle would be found. Then, both sides would be multiplied by the length of the hypotenuse and divided by the value of the cosine of the angle.
Work Step by Step
Cosine of an angle = adjacent sidelength/hypotenuse sidelength
$cos (x) = adjacent/hypotenuse$
$cos (x)*hypotenuse = adjacent/hypotenuse*hypotenuse$
$cos (x)*hypotenuse/cos (x) = adjacent/cos (x)$
$hypotenuse = adjacent/cos (x)$
