How do I find B using the boundary condition ux(0,t) = -Q in Laplace transforms?

[squenshl]

I'm deriving the heat equation using Laplace transforms.
I'm up to u(bar) = Ae^sqrt(s)x + Be^-sqrt(s)x.
My boundary conditions are lim(x tends to 0) = 0 & this makes A = 0, so u(bar) = Be^-sqrt(s)x
My other BC is u_x(0,t) = -Q
My question is how do I get B using this BC?
I tried using the def of Laplace transforms and get -dQ(bar)/dx

 

[squenshl]

I got B = Q/sqrt(s) and this implies that U(bar) = Qe^-sqrt(s)x/sqrt(s), I hope that's right