Question: Harita must memorize 90 measures of music for her cello solo at a concert. She plans on memorizing 18 new measures for every 3 days of practice. Which equation can be used to determine $m$, the number of measures Harita still needs to memorize, as a function of $d$, the number of days of practice since she began learning the piece?
1. **State the problem:** Harita has to memorize a total of $90$ measures. She memorizes $18$ measures every $3$ days. We want to find an equation for $m$, the number of measures left to memorize after $d$ days.
2. **Understand the rate:** She memorizes $18$ measures in $3$ days, so the rate of memorization per day is $$\frac{18}{3} = 6$$ measures per day.
3. **Write the equation:** The total measures to memorize is $90$. After $d$ days, she has memorized $6d$ measures. So the remaining measures $m$ is:
$$m = 90 - 6d$$
4. **Check the options:**
- $m = 90 - 6d$ matches our derived equation.
- Others do not match the rate or total measures.
**Final answer:** $$m = 90 - 6d$$