sandy can paint a house in 9 hours thus her rate is 1/9 house per hour

if alyssa helps it takes them 4 hours thus the together rate is 1/4 house per hour

let 'x' represent the number of hours that Alyssa needs to complete the job alone, x>0

1/x house per hour is the rate of Alyssa

1/9 + 1/x = 1/4

by solving we find:

x = 7.2 hr

x = 7 hr 0.2*60 min

x = 7 hr 12 min

click here to see the step by step solution of the equation:

Alyssa needs 7.2 hour or 7 hours and 12 minutes to complete the job alone.