lens problem

For the convex lens, use the thin lens formula 1/u + 1/v = 1/f
with u = ? v = 17 cm, f = 12 cm
1/u + 1/17 = 1/20
solve for u gives u = 40.8 cm

Hence, the "object" of the convex lens is situated at disance of 40.8 cm to the left of the lens, i.e. it is at distance of (40.8 - 30) cm = 10.8 cm to the left of the concave lens.

The "object" of the convex lens is the image formed by the concave lens, and image formed on the left side of the concave lens is virtual, Thus, applying the thin lens formula to the concave lens,
1/25 + 1/(-10.8) = 1/f
where f is the focal length of the concave lens
solve for f gives f = -19 cm