Let the number of hours Donnell practiced be d
and the number of hours Marcus practiced be m.
We can set up a system of equations.
d = m + 3
m + d = 11
Substitute the value of d that's in the first equation, into the second equation.
m + (m + 3) = 11
2m + 3 = 11
2m = 8
m = 4
Marcus practiced for 4 hours. Since Marcus and Donnell, together, practiced 11 hours, Donnell practiced for 11-4=7 hours.
