Hey there! I'm happy to help!
If the triangles are similar, the ratio between all of the sides is the same.
We see that to get the 4.2 in the big triangle to the 2.4 in the smaller triangle, we must divide by 1.75 (we get this from 4.2/2.4=1.75). So, if the ratio between all the sides is the same in both triangles, we can divide 6.3 by 1.75 to get y.
6.3/1.75=3.6
Therefore, the length of y is B. 3.6.
Have a wonderful day! :D
