Take into account, that in general, a cosine function of amplitude A, period T and vertical translation b, can be written as follow:
[tex]f(x)=A\cos (\frac{2\pi}{T}x)+b[/tex]
In the given case, you have:
A = 4
T = 3π/4
b = -3
By replacing you obtain:
[tex]\begin{gathered} f(x)=4\cos (\frac{2\pi}{\frac{3\pi}{4}}x)-3 \\ f(x)=4\cos (\frac{8}{3}x)-3 \end{gathered}[/tex]
Hence, the answer is:
f(x) = 4cos(8/3 x) - 3
