Let x represent the amount of sales volume for which the two offers result in the same pay.
Company A offers a weekly salary of $360 plus a commission of 18% on their dollar sales volume. This means that the total value offered by company A for x sales volume is
360 + 0.18x
Company B offers a weekly salary of $720 plus commission of 9% on their dollar sales volume. This means that the total value offered by company B for x sales volume is
720 + 0.09x
For the two offers to be the same, the sales volume would be