shift f(x)=6cos(3x+pi/2)
Let's find the phase shift of 6·cos(3x + π2)
For a·cos(bx + c) + d, write bx + c = b·(x + cb): the graph of a·cos(bx) + d is shifted horizontally by -cb
Here the angle is 3x + π2: b = 3 and c = π2, so the phase shift is -c/b = -π23 = -π6
-π6 < 0: the graph is shifted π6 to the left
▶Phase shift:   -π6