What is the greatest possible value of a in the system of equations 5a + 2b = 0 and ab = -10?

Acontractor needs 54 square feet of brick to construct a rectangular walkway. the length of the walkway is 15 feet more than the width. write an equation that could be used to determine the dimensions of the walkway. solve this equation to find the length and width, in feet, of the walkway.

How to do this , and can some one explain

Set $r$ is a set of rectangles such that (1) only the grid points shown here are used as vertices, (2) all sides are vertical or horizontal and (3) no two rectangles in the set are congruent. if $r$ contains the maximum possible number of rectangles given these conditions, what fraction of the rectangles in set $r$ are squares? express your answer as a common fraction.