Answer :
D:devaughn
S:sydney
D+S=83. First equation
D=S-9. Second equation
D-S=-9
Add 2 equations
D+S+D-S=83-9
2D=74
D=37
Devaughn is 37 years old
So
D+S=83
37+S=83
S=46
Sydney is 46 years old
S:sydney
D+S=83. First equation
D=S-9. Second equation
D-S=-9
Add 2 equations
D+S+D-S=83-9
2D=74
D=37
Devaughn is 37 years old
So
D+S=83
37+S=83
S=46
Sydney is 46 years old