To prepare a solution from different concentration stock, you need to make the content/mass of solute equal. The content is calculated by multiplying the concentration with the volume used.
The calculation would be:
content1= content2
V1 * M1= V2 * M2
V1 * 2.5M= 3.8L * 1.5 M
V1= 5.7/2.5= 2.28L = 2.3L
