Consider f(x)=5x\displaystyle {f{{\left({x}\right)}}}=-{5}{x}.

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

f(0)=\displaystyle {f{{\left({0}\right)}}}=  

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

f(st)=\displaystyle {f{{\left({s}-{t}\right)}}}=