Use "LaTex" code. There is a tutorial here: http://www.mathhelpforum.com/math-help/latex-help/.
You can see the code used for
, or any formula, by clicking on it.
Since is the identity matrix, as other have told you, A is just the "inverse matrix to .
There are a number of different ways to find the inverse matrix to a given matrix and, of course, we don't know which you have learned or are expected to use.
One very fundamental way would be to write that matrix equations as four linear equations: Writing A as we can write that matrix equation as
and, setting the same components equal, a+ 2c= 1, b+ 2d= 0, 3a+ 5c= 0, 3b+ 4d= 1, four equations in four values. (Actually, they can be separated- you really have two sets of two equation for two values which is a little easier.) Solve those for a, b, c, and d.