Answer:
sin a=-4/5
we have sina=perpendicularp/hypotenuseh=-4/5
let p=-4k
h=5k
by Pythagoras theorem
h²=p²+b²
(5k)²=(-4k)²+b²
25k²-16k²=b²
(3k)²=b²
b=√(3k)²=3k
now
cos a =b/h=3k/5k=3/5
