Answer :

[tex](5d - 7d^2t)^2=\\ 25d^2-70d^3t+49d^4t^2[/tex]
[tex](5d - 7d^2t)^2->(a-b)^2=a^2-2ab+b^2 \\ \\ (5d - 7d^2t)^2 \\ =5d*5d - 2*5d*7d^2t+7d^2t*7d^2t \\ =25d^2-70d^3t+49d^4t^2[/tex]