Given f(x)=6x2+5x5\displaystyle {f{{\left({x}\right)}}}=-{6}{x}^{{2}}+{5}{x}-{5} and g(x)=9x27x+2\displaystyle {g{{\left({x}\right)}}}=-{9}{x}^{{2}}-{7}{x}+{2}, find the constant a\displaystyle {a} such that:

(fg)(x)=54x43x32x2+ax10\displaystyle {\left({f}\cdot{g}\right)}{\left({x}\right)}={54}{x}^{{4}}-{3}{x}^{{3}}-{2}{x}^{{2}}+{a}{x}-{10}