A=(1,a;0,1/2) B=(1,-1;0,2) AB=I, calculate the value of a.

First notice I is the 2x2 identity matrix (1,0;0,1) now we can form an equation to solve for a, look for an entry where a is involved the (1,2) entry of I is 0 and calculated by 1(-1)+2a=0 now solving gives a=1/2