Think of the situation as Samantha having $\$1500$ minus $\$35$ times the number of weeks she has been spending. Use $x$ to represent the number of weeks she has been spending.

You want to know when she will have $\$915$ left, so set your expression equal to that amount. Then solve for $x$.

$1500−35x=915$

$16\frac{5}{7}$

$16$ weeks and $5$ days