A^2-4A-5I = 0 ==> A^2=5I+ 4A
A^2 is the inverse of aI+bA ==> A^2(aI+bA) = I
==> (5I+4A)(aI+bA)=I
==>4bA^2 + (5b+4a)A + (5a-1)I=0
==> 4b(5I+4A) + (5b+4a)A + (5a-1)I=0
==> (21b+4a)A + (5a+20b-1)I = 0
==> 21b+4a=0 and 5a+20b-1=0
==> a= 21/25 and b=-4/25
A^2 is the inverse of aI+bA ==> A^2(aI+bA) = I
==> (5I+4A)(aI+bA)=I
==>4bA^2 + (5b+4a)A + (5a-1)I=0
==> 4b(5I+4A) + (5b+4a)A + (5a-1)I=0
==> (21b+4a)A + (5a+20b-1)I = 0
==> 21b+4a=0 and 5a+20b-1=0
==> a= 21/25 and b=-4/25