Как указать от какой глагольной основы образованы формы повелительного наклонения??

Раскройте скобки,поставьте глагол в Past Continous

1)When the phone rang , she (write) a letter .

2)While we (have ) a picnic ,it started to rain.

3)Sally (work) when Joe had the car assident .

4)While John (sleep) last night ,someone stole his car.

5)Last night at 6 p.m.,I (eat) dinner .

6)I (study) while he (make) dinner.

7)While Ellen (read),Tim (watch ) television .

8)They (eat) dinner ,(discuss) their plans and (have) a good time.

решить систему уравнения

x^2+y^2=900,

xy=432