First make an equation system for the problem
An equation for "Jeanine is twice as old as her brother Marc" is
⇒ j = 2m
An equation for "the sum of their ages is 24" is
⇒ j + m = 24
Second solve the equations with subtitution method.
Subtitute j with 2m in the second equation
j + m = 24
2m + m = 24
3m = 24
m = 24/3
m = 8
Marc is 8 years old
Find Jeanine's age by subtitute m with 8 in the first equation
j = 2m
j = 2(8)
j = 16
Jeanine is 16 years old
