Since they are similar triangles, their sides are SIMILAR lengths....
One triangle has side length of 35 the other similar side in the OTHER triangle is 75 ft...
you can make a ratio of the side lengths 35 is to 75 35/75 is the ratio of similar sides in the two triangles
now they tell you DE is 21 feet.....and we are looking for the SIMILAR side in the other triangle, BA
these two sides will be in the same ratio 35/75
so you can say something like this 35 is to 75 as DE is to BA and we know DE is 21, so this looks like this:
35/75 = 21/BA now solve for BA ... can you do that now?