well first thing you need to kno is the derivative of sec which is 1/cos
for this problem you need to apply the dreaded chain rule. f'(g(x)) x g'(x)
the answer is
1/cos(y-x) x derivative of inside which is just y prime or y' (derivative of x is just 1) (note that the derivative of y is y prime only when your taking the derivative with respect to y but in this problem you are taking the derivative with respect to x)
1/cos(y-x) x y