If X and Y have the joint pdf
then find
find the conditional density of X given Y
First you must find the marginal of Y and divide the joint density by that marginal
When you finally have the conditional density, plug in your value for y and integrate x over the region.
HOWEVER, since the joint density is uniform over the triangle it will be uniform over the line y=.75
since you want 0<x<.5 the answer should be 2/3 which is two-thirds of that line.
DRAW the region and you will see how obvious this is.