Making Assumptions:

Drag force is equal to a function of radius, velocity, air density, and the viscosity of air.

Solves to:

L : a + b − 3c − d = 1,

T : −b − d = −2,

M : c + d = 1.

From the T equation we have b = 2−d, and from the M equation c = 1−d.

The L equation then gives us a = 2 − d

The book forms an equation using as an arbitrary number. I'm fine with that but, I can't figure out how they create the rest of the equation.

I conclude that a,b,c and d are substituted back into the equation but, I dont see how they get Rho on its own.

Can someone explain that to me?

Thanks.