multiple predecessor start-end

[zan1]

Hello.
I have been using project for 3 days and I have really some problems with multiple predecessors. Task 2 and 3 can start only after that task 1 has been completed. My data are: the end date of task 2 and 3 and their duration; the duration of task 1. (I don't know neither the start nor the end date of task 1). So the end date of task 1 should be equal to the minimum start date of task 2-3. I tried to impose two start-end constraints to task 1 with reference to task 2-3, but I got that task 1 end at the maximum start date of task 2-3.
How can I do?

Thanks.

 

[PDQBach]

Seems like a homework question to me. The trick is to set Project to use "Schedule from Finish Date".