...so why is u=+10 and not -10 ? As it will be in the direction opposite to the incident rays
According to the setup the image formed by 1st lens is real and it is 10 cm from the 2nd lens. But the rays after having refraction from 1st lens form a virtual image and hence this virtual image is on the left side of the 2nd lens and that is why they took -10 and not +10 for u2