The Chain Rule & Implicit Differentiation · Superchallenge problem

Let $u(x,t)$ be the solution of the wave equation $u_{tt} = 4u_{xx}$ on the whole line with $u(x,0) = x^2$ and $u_t(x,0) = x$. Using the change of variables $\xi = x + 2t$, $\eta = x - 2t$, find $u(1,1)$.