Equation of Motion of a Particle acted on by a retarding force

[physconomics]

Homework Statement

The equation of motion is $$\frac{d^2r}{dt^2} = a - y\frac{dr}{dt}$$
At time t = 0, $$r = r_0$$ and $$\frac{dr}{dt} = v_0$$
Show that $$d[a \times (yr + dr/dt]/dt = 0$$
and find the differential equation satisifed by $$s = a.r$$

Relevant Equations

Unsure

I really can't figure out where to even start on this question

 

[DEvens]

Can you fix the brackets in the expression you are to show is 0? Also, is "a" a constant? And is y a function of t?