We need to find a cosine function:
[tex]y(x)=acos(bx) \\ \\ where: \\ \\ \left|a\right|=Amplitude \\ \\ Period=\frac{2\pi}{b}[/tex]
The amplitude represents half the distance between the maximum and minimum values of the function and the period goes from the x-value of one peak to the x-value of the next one. Therefore:
[tex]a=0.75 \\ \\ b=\frac{2\pi}{10}=\frac{\pi}{5}[/tex]
Finally:
[tex]\boxed{y(x)=0.75cos(\frac{\pi}{5}x)}[/tex]
And y(4) is:
[tex]y(4)=0.75cos(\frac{\pi}{5}\times 4) \\ \\ \therefore \boxed{y(4)=-0.60}[/tex]
