How can I Calculate average velocity without time?

Will you settle for average speed ?

The general method for solving any problem is to use what you do know in order

to find the answer. Generally, you never use something you don't know, right ?

Well, you told us what you don't have, but neglected to mention what you do have.

If you know starting and ending speed, then average speed = 1/2 (starting speed + ending speed).

If you know distance and acceleration, then time = sqrt( 2 x distance / acceleration).

Then you have time and distance, and you can find average speed.