[tex]\displaystyle\bf\\\Big(\frac{5}{6} \Big)^{x^{2} -x} =\Big(\frac{5}{6} \Big)^{6} \\\\\\x^{2} -x=6\\\\x^{2} -x-6=0\\\\D=(-1)^{2} -4\cdot(-6)=1+24=25=5^{2} \\\\\\x_{1} =\frac{1-5}{2} =\frac{-4}{2} =-2\\\\\\x_{2} =\frac{1+5}{2} =\frac{6}{2} =3[/tex]