A 24.0 mL sample of an 5.7 % (m/v) KBr solution is diluted with water so that the final volume is 243.0 mL.?

Answer 1

C1V1 = C2V2

5.7% (24.0 mL) = C2 (243 mL)

C2 = 0.56%

Answer 2

Presumably you want the concentration of the diluted solution.  What units do you need?  M? m? m/v%? ppm? mole fraction? or something else?