let y=2x-8. What is the minimum value of the product xy?

Question

I'm not really sure how to do this...

I tried taking x*(2x-8), and the min is 0. but then i wanted to see if it was the same when solving for x, and i got a min of -8. How do i do this problem?

thanks, but i found which step i was missing.

thanks, but i found which step i was missing.