It appears that you correctly used the Pythagorean Theorem to find e, but you then found the value of $\displaystyle{\cot{{D}}}\usepackage{cmbright}\Huge\$ . Remember that the two acute angles in a right triangle will have different trigonometric ratios. The correct answer is $\displaystyle\frac{{4}}{\sqrt{{65}}}\usepackage{cmbright}\Huge\$ .