I have this assignment question which I am stuck on.

Ut + 2*Ux = -(U-20) with U(0,t) = 50 & U(x,0) = 20

I get; dt/1 = dx/2 = du/-(U-20)

From which I get b = t - (1/2)x

And U(x,t) = e^[-t-f(b)] + 20

For which I can only satisfy the initial condition U(0,t) = 50

I am struggling to understand what I am meant to do, please help.