The two sides of the two triangles PQR and STU are similar, that is, 6 and 4, which means that their 3rd side must also be similar. So, you equate the 3rd sides:
(3y - 2) = (y + 4), solve for y
3y - y =4 + 2
2y = 6
y = 3. Now, you must substitute this value of y into the 3rd side of PQR:
3y - 2 =3*3 - 2 = 7 ft - which is the 3rd side of PQR.
Then, Perimeter of PQR = 6 + 4 + 7 = 17 feet.