Trigonometric function question

If the terminal side of an angle G lies on the line
y = 3x; x < 0, then cos(G+pi/2)=???

y = 3x

x < 0

Find the cos(G) and the sin(G)

x = -1
y = -3

sqrt(x^2 + y^2) = sqrt(10)

cos(G) = x / sqrt(10) = -sqrt(10) / 10
sin(G) = y / sqrt(10) = -3sqrt(10) / 10

cos(G + pi/2) =>
cos(G)cos(pi/2) - sin(G)sin(pi/2) =>
(-sqrt(10)/10) * 0 - (-3sqrt(10) / 10) * 1 =>
3 * sqrt(10) / 10