Im a bit stuck on this one

Let X have a uniform ditribution on the interval (0,1). Given X=x, let Y have a (conditional) uniform distribution on the interval (0,2x)

a) Determine f(x,y), the joint pdf of X and Y.

b) Find http://www.sosmath.com/CBB/latexrend...3051d85f22.gif, the marginal pdf of Y

thanks