Explanation:
remember, tan(x) = sin(x)/cos(x)
the larger angle is between the 9 and 15 sides, as in a right-angled triangle the Hypotenuse (the side opposite of the 90° angle) is the longest side in the triangle.
and the bigger the acute angle the bigger the opposite side.
for this
12 = sine × 15
9 = cosine × 15
tangent = sine×15 / (cosine×15) = 12/9 = 4/3