Do all triangles have square corners?

When using the linear combination metho e linear combination method to solve a system of linear equations, the four options are:

If f(x) = x+8 and g(x) = 4x- 3, find (f+g)(x)

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.