That's correct, but again there is an equation to be found and solved :)

There are two points in time, now and then.

We define x as Jimmy's age now. "Then" is when Tom was x years old.

To get from now to then (reading Tom's line) we subtract 24 and add x, hence the bottom right cell is x-24+x = 2x-24.

Now the equation is simply 2 \times (2x - 24) = 24, or x = 18.