a store sells y+3 boxes of pencils. each box has 5y-7 pencils. how many pencils does the store sell?

Combine y+3 with 5y-7

Y+3=5y-7

Subtract 3 to both sides
Y=5y-10

Subtract 5y to both sides

-4y=10

Divide -4 to both sides

Y=-25