Urgent! A college probability topic, only one step will not! Please explain! Give extra points! thank you ! ! The problem is in the picture, I just don't understand it after analysis.

original formula = cov (3x, x)+cov (3x, 4y)+cov (-2y, x)+cov (-2y, 4y)

= 3d (x)+12cov (x, y)-2cov (x, y). Y) = cov (y, x)

cov (ax, by) = abcov (x, y)

cov (x, a) = , and a is a constant.