Ive got this DE: 8(du/dx)+4(du/dy)=9u anyone could help me?

I separate it with u=X(x)Y(y)

then I get: 8( X'(x)/X(x) =-4 ( X(x)Y'(y)) +9X(x)Y(y)

If it would be just: 8( X'(x)/X(x) =-4 ( X(x)Y'(y)) I could write = constant, but with that 9X(x)Y(y) how to solve?

Then I got conditions: u(0,y)=33*e^-y +50*e^4y

Please help me solve this.