$7\mathrm{.}27$ Given the following function:

$f(x,y)=-8x+x^2+12y+4y^2-2xy$

Determine the minimum $($a$)$ graphically, $($b$)$ numerically with the fminsearch function, and $($c$)$ substitute the result of $($b$)$ back into the function to determine the minimum $f(x,y).$