10y(9y + 21) ÷ 2(3y + 7)

Sure, let's break it down:

Step 1: Distribute the 10y into the parentheses (9y + 21) This gives us 90y^2 + 210y

Step 2: Distribute the 2 into the parentheses (3y + 7) This gives us 6y + 14

Step 3: Now we have the equation 90y^2 + 210y ÷ 6y + 14

Step 4: Divide each term in the numerator (90y^2 + 210y) by each term in the denominator (6y + 14)

This gives us (90y^2 ÷ 6y) + (210y ÷ 14)

Step 5: Simplify each term

This gives us 15y + 15

So, 10y(9y + 21) ÷ 2(3y + 7) simplifies to 15y + 15.