Question

( a -4b 5
6 -a+b )= ( 11 5
6 -5 )

Answer 1

$\left[\begin{array}{ccc}a-4b&5\\6&-a+b\end{array}\right] = \left[\begin{array}{ccc}11&5\\6&-5\end{array}\right]$

comparing, 
a-4b=11
-a+b=-5

so -3b=6  =>b=-2
so a=b+5=3