Okay, so you know that A pairs with T, and C pairs with G.
So if there are 19 C bases in Strand B, there will be 19 G bases in Strand A, and vice versa.
There are 9 A bases in strand B so there will be 9 T bases in Strand A.
Therefore 9+19+26=54
In the question you are told there are 74 bases in total, so 74-54=20.
So there are 20 A bases in Strand A, and 20 T bases in Strand B.