Question 836607
Basically you insert (a-4) into x so.... 

{{{(a-4)^2-7(a-4)}}} --> {{{a^2-16-7a + 28}}} --> {{{a^2-7a+12}}} 

Then we solve the quadratic equation: 

*[invoke quadratic "a", 1, -7, 12 ]

Ultimately, the answer is 4 and 3.