task relationships

Firstly I want to apologise if this is the wrong place to ask this type of question. I am new to this forum and new to cProject.

I am trying to create a relationship between two task so that the first task has to start 90 days before the second. The second task is the driver for this and it is not a case of starting task 2 90 days after task 1. What ever date is set for task 2 needs to be set 90 days before in task 1. I hope this is clear. I look forward to hearing from some one out there.

Thanks in advance.

Steve