First you have to get rid of the number in front of theta. To do this divide by 3/4. After you do this you should get

y=100Cos3/4(θ+4π/12) You can reduce 4π/12 to π/3

so now amp=100; period= 2π/(3/4) = 8π/3; and phase shift = -π/3

Because you're using Cos, start off at the max amp then move down with each period.

You will have (100, -π/3), (0, 7π/3), (-100, 15π/3 (which is reduced to: 5π)), and so on.

If you were confused about any steps please ask.