<span>x(t-u) = 3t
dividing both sides by (t-u):
So.. </span><span><em>x = 3t/(t-u)</em></span>

First, simplify

to

/ Your problem should look like:
Second, multiply both sides by 36. / Your problem should look like:
Third, multiply both sides by -1. / Your problem should look like:
Fourth, switch your sides. / Your problem should look like:

Answer:
C) 21