Let J be the number of marbles owned by John, and S the number of marbles owned by Steve.
John has three more marbles than Steve, so [tex] J = S + 3 \iff J - S = 3 [/tex]
They have 25 marbles, so [tex] J+S = 25 [/tex]
So, we have the following system:
[tex] \begin{cases} J - S = 3\\J + S = 25\end{cases} [/tex]
Add the two equations side by side:
[tex] (J - S) + (J + S) = 3 + 25 \iff 2J = 28 \iff J = 14 [/tex]
Since John has three more marbles, we have
[tex] S = J - 3 = 14 - 3 = 11 [/tex]
