Solusi - Masalah Kata

2 sentences contain the information we are after.

The first, [The difference between the 2 numbers is 3], we learn that:

There are 2 numbers we are looking for, as we don't know what they are, we will refer to them as $$x$$ and $$y$$.

The difference of $$x$$ and $$y$$ is 3. This can be mathematically expressed as $$x-y=3$$

A partir de la siguiente oración, [the sum of the 2 numbers is 21], aprendemos que la sum de $$x$$ y $$y$$ es igual a 21.

Esta información puede entonces ser expresada en otra ecuación, que sería $$x+y=21$$

Ahora tenemos un sistema de ecuaciones:

$$x-y=3$$
$$x+y=21$$