If this is simple proportion then y=kx and we find the constant k by putting y=12 and x=15: 12=15k, k=12/15=4/5.
Now we have y=4x/5, so if y=21, 21=4x/5, 4x=105, x=105/4=26¼.
If this is inverse proportionality, xy=c and we plug in the x, y values to find c:
15×12=180=c. Therefore xy=180, so if y=21, 21x=120, x=120/21=40/7=5⁵⁄₇.
There is no indication in the question of how terminal velocity comes into this. We don’t know the significance of x and y in this context.