Please see the work below.
Work Step by Step
We know that: $M_1V_1=M_2V_2$ Where $M_1$ is the concentration of the original solution and $V_1$ is the volume of the original solution. $M_2$ and $V_2$ are the concentration and final volume of the diluted solution, respectively. We plug in the known values into the above equation to obtain: $25mL\times 0.86M=500mL\times M_2$ $M_2=23.25M$