After Essie Basham dies, John-Boy says, "I got the circuit preacher to take care of everything." A circuit preacher is a Christian minister who, due to a shortage of ministers, officiates at multiple churches in an area. Even in the 21st century it's still common for small rural congregations to share clergy.
Mary Ellen asks when she will use algebra in nursing. It would have most commonly used to figure correct medication dosages (as many were in powders, even liquids, & not yet in dosage tablets, though even those were limited & often had to be crushed & calculated). Correct dosages had to be calculated based, most commonly, on patient's weight. The doctor would order the medication & dose, it would be up to the nurse to portion out proper dosage before aministering it.