We know that sin²x+cos²x=1 so clear cos x cos x=(+/-)√[1-sin²x]
in this problem <span>Angle 0 is in quadrant 1 -----> cos o and sin o are positive </span>sin o=2/5 cos x=√[1-(2/5)²]----> cos o=√[1-4/25]----> cos o=√[21/25]---> cos o=√21/5
The line through that point and the origin has a slope of -1. It is perpendicular to the line you want, so the line you want has slope +1. Its equation can be written as .. y = (x -4) -4 or, in standard form .. x - y = 8