past test 2019 day problem 3B

[Yan Zhou]

I think there are some typo in the answer of u(x,t).
$$u(x,t) =  6sin(x+3t) +  6sin(x-3t) $$ as $x>3t$,
$$u(x,t) = 6sin(x+3t) + 6sin(2(x-3t))$$ as $t<x<3t$.