This was not just easy, it was Dan Quayle easy, so my grading was easy.
The easiest solution: Step 1. equation of characteristics:
$$
\frac{dx}{3}=\frac{dt}{2}\implies x-\frac{3}{2}t =C \implies u=f(x-\frac{3}{2}t)
$$
is a general solution. Other equivalent forms are possible leading to the same final answer, but this one is the most natural and straightforward.
Step 2 Initial condition: $u(x,0)=f(x)=\sin(x)$ and therefore
$$
\boxed{u(x,t)=\sin(x-\frac{3}{2}t)}.
$$
Several students put the wrong sign $u=\sin(x+\frac{3}{2}t)$, several made mistakes on Step 2 and got marks halved. Few made really grave mistakes like trying method of separation, but majority did well and got all 20 (correct but ugly solutions/answers are not punished).
