Question 23890
Before you can "solve" this, it needs to be an "equation" rather than an "expression". Set your expression = 0 and try solving by factoring.
{{{a^2-11a+28 = 0}}} Factor.
{{{(a-4)(a-7) = 0}}} Apply the zero product principle.
{{{a-4 = 0}}} and/or {{{a-7 = 0}}}
If {{{a-4 = 0}}} then {{{a = 4}}}
If {{{a-7 = 0}}} then {{{a = 7}}}