The sine function is an odd function, that is, it has the following property:
[tex]f(-x)=-f(x)[/tex]Using this property, we can find the value of f(-a):
[tex]\begin{gathered} f(a)=\frac{3}{4} \\ f(-a)=-f(a)=-\frac{3}{4} \end{gathered}[/tex]Now we can calculate the value of the expression:
[tex]f(a)+5f(-a)=\frac{3}{4}+5\cdot(-\frac{3}{4})=\frac{3}{4}-\frac{15}{4}=-\frac{12}{4}=-3[/tex]