The sine of an angle in a right triangle is the length of the leg farthest from that angle divided by the length of the hypotenuse of the right triangle.
The angle is 15 degrees. The leg farthest from that angle has length 12. The hypotenuse has length x.

Simplify this equation such that we only have x isolated on one side of the equation.
Dividing both sides by
and multiplying both sides by x gives the following the equation.

This is the expression.