Chromatographic analysis of a solution which contained 3.87 mM analyte (X) and 1.82 mM standard (S) produced peak areas of 4225 and 13203 respectively.
a) Calculate the response factor for the analyte
b) Next, 2.00 mL of 9.276 mM S was added to 10.00mL of a solution of unknown concentration of X, and the mixture was diluted to 25.0 mL. Chromatographic analysis of the solution produced peak areas of 5366 for X and 4615 for S. What was the concentration of X (in mM) in the unknown solution before the addition of standard and dilution?
Help with b please?

Question

Chromatographic analysis of a solution which contained 3.87 mM analyte (X) and 1.82 mM standard (S) produced peak areas of 4225 and 13203 respectively. 
a) Calculate the response factor for the analyte
b) Next, 2.00 mL of 9.276 mM S was added to 10.00mL of a solution of unknown concentration of X, and the mixture was diluted to 25.0 mL. Chromatographic analysis of the solution produced peak areas of 5366 for X and 4615 for S. What was the concentration of X (in mM) in the unknown solution before the addition of standard and dilution?
Help with b please?