I can't give an exact answer if you do not provide any unit of concentration. The most common unit of concentration is molarity, which is the number of moles of solute over the number of liters solution. Say for example, the concentration of the solution is 0.1 M. To find the equivalent mass percentage, the solution is as follows:
The molar mass of Sr(NO₃)₂ is 211.63 g/mol.
(0.1 mol Sr(NO₃)₂/ L solution)(211.63 g/mol)(1 L solution/ 1 kg solution)(1 kg/1000 g solution) * 100 = 2.12% Sr(NO₃)₂ solution