math
posted by emily .
how to solve y= 3cos2x + 5
pls help me

Is that really cos(2x) or do you mean cos^2 x?
I will assume that you really do mean cos(2x). In that case
cos(2x) = cos^2 x  sin^2 x
= 2 cos^2 x 1
y = 6 cos^2x 3 + 5 = 6 cos^2 x  2
cos^2 x = (y+2)/6
cos x = sqrt[(y+2)/6)]
You can use that to solve for x, if that is what the question is all about.
You cannot solve a single equation for both x and y.