y=4cos(3x-2(3.14))
I will assume that:
(1) You want the value of x
(2) x is in radians
(3) 3.14 represents pi, which is somewhat more (3.14592..)
There are two unknowns in this single equation, so we can only solve for x as a function of y.
With those assumptions,
y = 4 cos 3x
cos 3x = y/4
3x = arccos(y/4)
x = (1/3) arccos(y/4)