In order to solve this question, you use the formula C1V1=C2V2; C1 and V1 represent the concentration and volume of the starting solution respectively, and C2 and V2 represent the concentration and volume of the final solution respectively.
In this case, C1 is 10.5, V1 is unknown, C2 is 0.98 and V2 is 3 liters.
(10.5)(x)=(0.98)(3) (divide both sides by 10.5)
X=0.28 L
This means that you will use 0.28 L of the 10.5 M solution and 2.27 L (3 L-0.28 L) of water to make 3 L of a 0.98 M solution.
Hope this helps
