$a)f\left(x\right)+g\left(x\right)=\frac{3}{4}{x}^6-2x^4+\frac{5}{4}{x}^2+4+\frac{1}{4}{x}^6+2x^4+x^3-\frac{1}{4}{x}^2-2x+2=$

$=x^6+x^3+x^2-2x+6$