$4-6(15$ points$)$

Given two sets $S1$ and S$2($each of size $n),$ and a number $x,$ describe an $O(nlogn)$ algorithm for finding whether there exists a pair of elements, one from $S1$ and one from $S2,$ that add up to $x.($For partial credit, give a $(n2)$ algorithm for this problem.$)$