This is basically saying how much of the pizza did Tashia and Jay both ate, thus combining their portions together.
1/3 + 3/8 = fraction of pizza eaten
To add fractions, you must have a common denominator. Which is 24 in this case because 3*8 = 24.
So you multiply 1/3 with 8 and multiply 3/8 with 3.
1/3*8= 8/24
3/8*3= 9/24
Now you add the fractions.
8/24+9/24= 17/24. You add the numerator, the denominator stays the same.
17/24 of the pizza was eaten.