Hi there!
The general formula to find the density of a substance is :
Density = Mass ÷ Volume
Since you have the density and the volume, you can replace those information in the formula and solve the equation by isolating the mass :
0.876 = Mass ÷ 25.6
Multiply each side of the equation by 25.6
22.4256 = Mass
Your answer is: The mass of a substance is 22.4256g.
There you go! I really hope this helped, if there's anything just let me know! :)