It means linear momentum is conserved along X -axis. According to the law of conservation of linear momentum, pi=pf mAuA+mBuB=mAvA+(mBvB‌cos‌θ) =mAvA+mB1vB1‌cos‌90∘+mB2vB2‌cos‌θ ⇒10×10√3+20×0=10×0+10×10×0+10×20‌cos‌θ ⇒‌‌10×10√3=200‌cos‌θ where, cos‌θ being the horizontal component i.e., along X-axis ⇒‌‌cos‌θ=√3/2 ⇒‌‌θ=30∘‌‌[∵cos‌30∘=√3/2]