Explanation:
Since alpha a beta are in the first quadrant, the values of cos and sin for them are positive.
Trig identities
sinΦ ^2 + cos Φ^2 = 1
Using this identity, then cos a = 15/17 cos b = 12/13
Next , sub in the values using this trig identity:
sin ( a - b ) = sin a cos b - cos a sin b
= (8/17) (12/13) - (15/17)(5/13) = 21/221