To expand a little on what princeps showed you, if you plot the point (9, 12) in the coordinate plane, and drop a perpendicular to the x-axis and a line back to the origin, you will have formed a right triangle. You will note that x = 9 and y = 12.
You need to find r, the length of the hypotenuse. Use the Pythagorean Theorem. . We determine that r = 15.
Since we know that sin of an acute angle in a right triangle is the ratio of the opposite side of the angle divided by the hypotenuse, we can say: