(click on that to see the LaTex code I used.)
Since equality for matrices is defined to mean that all corresponding entries are equal, that tells you that a+ b= 7, 2a- 8= 2, a- b= 3, and 0= 0. Since that is three equations ("0= 0" is always true, of course) for two unknown values, it may be "over determined", that is, there may not be 2 numbers that satisfy all three equations.
I recommend starting by adding the first and third equations. That will immediately eliminate b and you can solve for a. The use any one of the equation to find the value of b for that a. Finally, check that all equations are satified.