Given that f(x)=x27x\displaystyle {f{{\left({x}\right)}}}={x}^{{2}}-{7}{x} and g(x)=x+15\displaystyle {g{{\left({x}\right)}}}={x}+{15}, find and simplify the following:


(a) (f+g)(x)\displaystyle {\left({f}+{g}\right)}{\left({x}\right)}=  
(b) (fg)(x)\displaystyle {\left({f}-{g}\right)}{\left({x}\right)}=  
(c) (fg)(x)\displaystyle {\left({f}{g}\right)}{\left({x}\right)}=