"At least" refers to being greater than or equal to (≥).
This is because if you have "at least" x of something, you have no less than x. You might have exactly x, or you might have more, but no less.
So when something is "at least" 15 inches long, that means it can be written in the inequality x ≥ 15.
In this case, that means Tina's hair length ≥ 15 inches.
Let's use the variable x for the length of her hair, though.
So the answer is x ≥ 15 inches.
Hope this helps!