To find the measure of angle \( C \) in the right triangle \( ABC \), where \( B \) is the right angle, we can use the formula for the tangent of angle \( C \):
\[ \tan(C) = \frac{\text{opposite}}{\text{adjacent}} \]
In this triangle, the side opposite angle \( C \) is side \( AB \) (which has length 9), and the adjacent side is \( BC \) (which has length 12). Thus we have:
\[ \tan(C) = \frac{AB}{BC} = \frac{9}{12} = \frac{3}{4} \]
Now, to find angle \( C \), we take the inverse tangent (arctangent) of \( \frac{3}{4} \):
\[ C = \tan^{-1}\left(\frac{3}{4}\right) \]
Calculating this using a calculator or trigonometric tables:
\[ C \approx 36.87° \]
Rounding to the nearest whole degree gives us:
\[ C \approx 37° \]
Thus, the approximate angle measure of vertex \( C \) is:
37°