Cos B = x / r
r = sqrt (x^2 + y^2 ) = sqrt [ (-4)^2 + (3sqrt(5))^2 ] = sqrt [ 16 + 45 ]= sqrt (61)
So
Cos B = -4 / sqrt (61) = -4[sqrt (61) ] / [ sqrt (61) * sqrt (61) ] = [-4 sqrt (61) ] / 61
