Deriving from Doppler's formula

Question

I am studying Doppler sonography. I do understand doppler's formula: $$ f=f_{0}\frac{v}{v-u}, $$ where $f_{0}$ is emitted frequency, f is received frequency, v is speed of the ultrasound puls and u is speed of blood from which is the ultrasound puls bouncing back. But I am struggling with how to get from above formula to this: $$ \Delta f = f-f_{0}=2f_{0}u. $$ Can anyone one give me hint? Thank you.

Image I am working with