The following is just a guess because I'm not sure what the problem is.
[tex] (0.5n^5)^2(10n^7)^3 = [/tex]
[tex] = 0.5^2 \times (n^5)^2 \times 10^3 \times (n^7)^3 [/tex]
[tex] = 0.25 \times 1000 \times n^{10} \times n^{21} [/tex]
[tex] = 250n^{31} [/tex]
