You can complete the square onanyquadratic - even the quadratic formula is derived by completing the square. I suspect they want you to complete the square because the only other way is to find and then check the sign of . In other words calculus which is not lower than A level.

Completed the square form is . The lowest point on the graph is equal to because

into completed square form is .

Aligning that with the form we get and

In your case you have , and

My answer is in the spoiler but if you have any issues getting there please post your workings

Spoiler: