`a)`
`f(x) = 5+3x^2 -x -2x^2`
`⇒f(x) = 5+x^2 -x`
`g(x) = 3x+ 3 - x- x^2`
`⇒g(x) = 2x+ 3- x^2`
`b)`
`H(x) =f(x) + g (x)= 5+x^2 -x+ 2x+ 3- x^2`
`⇒H(x)=2x+8`
`K(x) = g(x) -f(x)=5+x^2 -x-( 2x+ 3- x^2)`
`⇒K(x)=-2x^2+3x-2`
`c)`
`A(x)= x^2-4x=0`
`⇔x(x-4)=0`
⇔\(\left[ \begin{array}{l}x=0\\x-4=0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x=0\\x=4\end{array} \right.\)
`A(x)= x^2+4x=0`
`⇔x(x+4)=0`
⇔\(\left[ \begin{array}{l}x=0\\x+4=0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x=0\\x=-4\end{array} \right.\)