Prove that : $\rho(x,y)= \sqrt{(x_1-y_1)^2+4(x_2-y_2)^2} $ is a metric

let $X= R^2$. If $x$ and $y$ are points in the plane and their cordinates are $x=(x_1,x_2), y=(y_1,y_2)$ prove that the given function is a metric.

I am currently stuck on proving the triangular difference, any idea on how to prove it ?