Answer
The diagonals are congruent.
Work Step by Step
The Length of $\overline{PN}$ can be found as:
$d =\sqrt {(x_2-x_1)^2+(y_2-y_1)^2}=\sqrt {(-2a-a)^2+(0-b)^2}=\sqrt {(-3a)^2+(-b)^2}$
The Length of $\overline{MR}$ can be found as:
$d =\sqrt {(x_2-x_1)^2+(y_2-y_1)^2}=\sqrt {(-2a-a)^2+(0-b)^2}=\sqrt {9a^2+b^2}$
We can see that the diagonals are congruent because their respective lengths are equal.
