Explanation:
rc = planted rows of carrots
rt = planted rows of tomatoes
h = number of hours
rc = 4h + 1 (starting with 1 already finished row there will be 4 additional rows every hour)
rt = 3h + 8 (staying with 8 already finished rows there will be 3 additional rows every hour).
when will they have planned the same number of rows ?
when rc = rt, of course.
so,
4h + 1 = 3h + 8
h = 7
4×7 + 1 = 28 + 1 = 29 rows
after 7 hours Quincy and his mom will each have planted 29 rows of vegetables.