Solve for y. triangle pqr is congruent to triangle def, pq= 15, qr= 10, rp=8, and ef = 6y +4

Congruent means that they are identical
 looking at the order of the letters in the triangles EF are the last 2 and QR are the last 2
 so EF should equal QR

we are told QR = 10 so EF needs to equal 10
 so we have 6y+4 = 10

now solve for Y

6y +4 = 10
 subtract 4 from each side:
6y = 6
 divide each side by 6
y = 1