let

t = the hours Tanner needs to to grade the projects alone

1/t projects per hour is the rate of Tanner

j = the hours Joel needs to to grade the projects alone

1/j projects per hour is the rate of Joel

Working alone, it would take Tanner 2 hours longer than it would take Joel to grade the projects:

t = 2 + j

Tanner and Joel can grade their students' projects in 2 hours thus their together rate is: 1/2 projects per hour

1/t + 1/j = 1/2

plug t = 2 + j into the last equation:

1/(2 + j) + 1/j = 1/2

by solving we find:

j = 3.24 hr approximately

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it would take 3.24 hr approximately for Joel to grade the projects alone.