let

h = the hour Heidi need to clean the garage alone thus her rate is 1/h garage per hour, h>0

s = the hours Sarah need to clean the garage alone thus her rate is 1/s garage per hour, s>0

Sarah and Heidi can clean the garage in 2 hours

1/h + 1/s = 1/2

If they work alone, it takes Heidi 3 hours longer than it takes Sarah

h = s + 3

plug h = s + 3 into 1/h + 1/s = 1/2

1/(s + 3) + 1/s = 1/2

by solving we find

s = 3 hr

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h = 3 + 3

h = 6 hr

Heidi will clean the garage alone in 6 hours.