Hi there! The answer is n = 10.
[tex](2 {x}^{9} {y}^{n} )(4 {x}^{2} {y}^{10} ) = 8 {x}^{11} {y}^{20} [/tex]
As you see at the powers of x, we need to add the exponents of the power we when multiply them.
[tex] {x}^{9} \times {x}^{2} = {x}^{9 + 2} = x {}^{11} [/tex]
The powers of y work the same way.
[tex] {y}^{n} \times {y}^{10} = {y}^{n + 10} = {y}^{20} [/tex]
Hence, n = 10, since
[tex] {y}^{10 + 10} = {y}^{20} [/tex]
