calculus 3 math problem i dont understand:(?

V = ∫∫ (7+x)(13+y) dA
..= ∫(r = 0 to 6) ∫(θ = 0 to 2π) (7 + r cos θ)(13 + r sin θ) * (r dθ dr), via polar coordinates
..= ∫(r = 0 to 6) ∫(θ = 0 to 2π) (91r + 7r^2 sin θ + 13r^2 cos θ + r^3 sin θ cos θ) dθ dr
..= ∫(r = 0 to 6) (91rθ - 7r^2 cos θ + 13r^2 sin θ + (1/2) r^3 sin^2(θ)) {for θ = 0 to 2π} dr
..= ∫(r = 0 to 6) 182πr dr
..= 91πr^2 {for r = 0 to 6}
..= 3276π.

I hope this helps!