The side of the little square is s and the side of the big square is s+3
The area of the little square is s2 and of the big square is s2+6s+9.
The total area of the 2 squares combined is 549 (NOTE: I am going to assume your question has a exponent in error!!!!).
Therefore 2s2+6s-540=0
s2+3s-270=0
The quadratic formula gives an extraneous rot and x=15.