This is a standard question of enlargement.
We see that triangle DJY has one length of 42, and another or 50.
Triangle FJS has one length of (63+42=)105 and another of 50+x. We want to find out what x is.
Side YJ of the smaller triangle has a length of 42, and its corresponding (matching) length on the larger triangle, SJ, has a length of 105. We can use this information to find out the scale factor.
SJ -> YJ (larger to smaller):
105÷42=2.5
We're doing this next step as you will want to convert the length on the smaller triangle to one on the larger triangle in order to help find x.
YJ -> SJ (smaller to larger):
42×2.5=105
So, the scale factor is 2.5.
We now need to find FJ from DJ:
50×2.5=125
FJ is 125.
On the diagram, it is clear that FJ-DJ=FD. So, let's substitute our driven and discovered lengths:
125-50=75
Therefore, FD (a.k.a 'x') is 75.