It's asking to solve for a1 but I don't really know how, please help.

Acollege is planning to construct a rectangular parking lot on land bordered on one side by a highway. the plan is to use 680 feet of fencing to fence off the other three sides. what dimensions should the lot have if the enclosed area is to be a maximum?

What is the missing step in this proof ?

Plz hurry evaluate the expression a+b where a=8 and b=19