普通恒等式問題

first of all, re-arrange the variables in correct form:
-2(10s+3t)(3t-10s)
= -2[(3t+10s)(3t-10s)]
= -2[(3t)^2 - (10s)^s]
= -2[9t^2 - 100s^2]
= 200s^2 - 18t^2

can't see what's no. 2

for no.3:
3(-7h-2k)(2k-7h)
= 3{[(-7h)-2k][(-7h)+2k]}
= 3[(-7h)^2 - (2k)^2]
= 3[49h^2 - 4k^2]
= 147h^2 - 12k^2

簡單來說，回答發問同學的問題，錯誤在於寫了
(3t - 10s) 為 (10s - 3t)。

事實上
(3t - 10s) ≠ (10s - 3t)

正確的是
(3t - 10s) = -(10s - 3t)　〔有一個負號〕