Answer: x = (r - q) / p and px = r - q
Explanation:
Subtract q from both sides that you are having but if they are positive or if they are negative you have to add q to both sides and that is px + q - q = r - q, which simplifies to px = r - q.
Dividing by p on both sides that you are having and that is px / p = (r - q) / p, which simplifies to x = (r - q) / p.