If $$x^4 + y^4 + x^2y^2 = 21$$ and $$x^2 + y^2 - xy = 7$$, then what is the value of $$\frac{x}{y} + \frac{y}{x} ?$$

A

$$\frac{5}{4}$$

B

$$\frac{3}{4}$$

C

$$-\frac{3}{2}$$

D

$$-\frac{5}{2}$$