Graphing a differential equation in mathematica?

I'm trying to graph a first order differential equation in Mathematica but the graph doesn't seem right when compared to my txt book. I'm using NDSolve.

The first order diff-eq is...

dy/dt' = -cy-sin(x)+fcos(z)

where z = omega t'

c = 0.05

f = 0.4

omega = 0.7

Time t' is the x-axis

and dx/dt' is the y-axis

I have...

s = NDSolve[{\[Omega] x'[z] ==

y[z], \[Omega] y'[z] == -c y[z] - Sin[x[z]] + f Cos[z],

y[0] == -1, x[0] == 0}, {x, y}, {z, 0, 30}]

Plot[Evaluate[y[z] /. s], {z, 0, 30}, PlotRange -> All]

the graph should look akin to a cos function but the peaks are off compared to what's in my txt book.

Any ideas?