A cosine function has the form
[tex]y=A\cdot\cos (Bx+C)+D[/tex]Where A is the amplitude, B is 2pi/T, and C is null in this case because the phase is not being specified, and D is the vertical shift (midline).
Using all the given information, we have
[tex]y=3\cdot\cos (\frac{2\pi}{T}x)+4[/tex]Then,
[tex]y=3\cdot\cos (\frac{2\pi}{\frac{8}{5}}x)+4=3\cdot\cos (\frac{10\pi}{8}x)+4=3\cdot\cos (\frac{5\pi}{4}x)+4[/tex]Hence, the function is
[tex]y=3\cos (\frac{5\pi}{4}x)+4[/tex]