For this question we are using a formula that will show us what happens to a substance when it undergoes dissolution, the formula is:
M1*V1 = M2V2
Where:
M1 = initial concentration, 3*10^-4 M
V1 = initial volume, 0.2 mL
M2 = final concentration, ?
V2 = final volume, 10 mL
Now we add the values into the formula:
3*10^-4 * 0.2 = M2*10
6*10^-5 = 10M2
M2 = 6*10^-6, or 0.000006 M, this will be the final concentration