Don’t understand b) pls explain

Answer:
−s2+2st−t2−s+t
Step-by-step explanation:
−(s−t)−(s−t)2
distributed would be
−s+t+−s2+2st+−t2
Answer:
Below in bold.
Step-by-step explanation:
-(s - t) - (s - t)^2 Take out (s - t):
= (s - t)( -1 - (s - t)
= (s - t)(t - s - 1). <---- Factored form.
Or, if you want to expand the expression:
-(s - t) - (s - t)^2
= - s + t - (s^2 - 2st + t^2)
= -s^2 + 2st - t^2 - s + t.