Create a system of equations to solve

roberto is twice as old as katie.
in 4 years, roberto will be eight years older than katie.

how old is roberto? roberto is old.

how old is katie? katie is old.

R=2K which equals 2R=4K
4(R=K+8) or 4R=4K+32

Subtract first equation from second equation.
2R=32 so R=16

Plug into first equation
16=2K so K=8

Now plug both into second equation to check
4R-4K=4(R-K)=4(8)=32 it works

So Roberto is 16 and Katie is 8
(In 4 years R would be 20 and K would be 12 which has a difference of 8)