Consider f(x)=9x+12\displaystyle {f{{\left({x}\right)}}}=-{9}{x}+{12}

f(9)=\displaystyle {f{{\left(-{9}\right)}}}=  

f(6)=\displaystyle {f{{\left({6}\right)}}}=  

f(a)=\displaystyle {f{{\left({a}\right)}}}=  

f(a+b)=\displaystyle {f{{\left({a}+{b}\right)}}}=