Let us check the transformations. The first step is regarding the argument of the function, the -2x part. So, first of all, the minus sign implies that the function is reflected along the y-axis since f(x) is replaced with f(-x). However, cosx is symmetric along that axis so there is no change on the graph. Also, the 2 factor means that the function is compressed along the x-axis, since now f(2) corresponds to f(1) etc. (if we substitute 1 in the cos(2x), it is as if substituting 2 in the origninal function cosx). Finally, we have that the factor 3 in front of cos, implies that the function is dilated along the y-axis; the highs become 3 times higher and the lows 3 times low.
