Explanation:
sin∅=-3/4 in 4 quadrant which sine is negative there
using soh cah toa
sin=opp/hyp
where our opp=-3 and our hyp=4
using pythagoras theorem
hyp²=opp²+adj²
(4)²=(-3)²+adj²
16=9+adj²
16-9=adj²
adj²=7
adj=√7
from soh cah toa
cos∅=adj/hyp
cos∅=√7/4