Hello,
Let be the area of the rectangle.
What you look for is
So find the derivative for and find its value at t=64.
Another consequence of not quoting the post you are replying to is this, the blighter has deleted his/her question leaving your answer hanging. This makes it more difficult for someone else to learn from what you have done.
(Quoting also makes it more difficult for someone who is trying to cheat in maths competions or on their exams to elliminate the evidence)
RonL