Let
d -----> ages of Devon
m ---> ages of Milan
we have that
d+m=48 ------> equation 1
d=m+36 -----> equation 2
Solve the system of equations
substitute equation 2 in equation 1
(m+36)+m=48
solve for m
2m=48-36
2m=12
m=6
d=6+36
d=42
therefore
Devon is 42 years old
Milan is 6 years old