This question is talking about a dilution and dilutions are described using the equation M1V1=M2V2. The Ms represent molarities and the Vs represent volumes while the variables with a 1 attached to it are initial conditions which the 2s are final conditions. In this problem you are being asked for initial volume and the other three variables are given to you as follows:
M1=3 M
M2=.2 M
V2= 125 mL
Plugging those into the equation gives (3)V1=(125)(.2). Solving algebraically for V1 gives V1=(125)(.2)/(3)= 8.3 mL
