Application of Euler Cromer algorithm to coupled differential equations

Question

I have a situation where a tennis ball is hit at an angle Θ₀ from some height y₀ above the ground. The equations representing this motion are :

dv_x / dt = -k v (C_D v_x - C_L v_y) ;

dv_y / dt = - g - k v (C_L v_x + C_Dv_y).

Here v_x = v cos Θ and v_y $= v sin Θ$ , C_D is drag coefficient and C_L is lift coefficient.

To solve these equations using Euler-Cromer algorithm I need two functions $g(t,x)$ and $f(t, v)$ , such that

$$\frac{dv}{dt} = g(t, x) ; \frac{dx}{dt} = f(t, v)$$.

I really do not know how to reduce these two equations to the required functions (being a coder i am not very good at mathematics). The link to related article is : Calculations of ground stroke trajectories