Begin by setting up the equation for terminal velocity, which is as follows: V = sqrt ( (2 * m) / (Cd * r * A)) , where V is equal to terminal velocity, m is mass of the object, Cd is the drag coefficient, r is the atmospheric density and A is the projected, or reference, area of the object.
Square both sides of the equation to eliminate the square root on either side. This will leave you with the following equation: V^2 = (2*m)/(Cd*r*A). Doing this will make it easier to separate m, or mass, from the full equation.
Multiply both sides of the equation by (Cd*r*A), which will leave the equation as follows: (V^2)(Cd*r*A) = 2m.
Divide both sides of the equation by 2 to finally isolate m, or mass, on its own. This will leave you with the following equation: ((V^2)(Cd*r*A))/(2) = m.
Plug in the variables for your equation, aside from mass, and solve. The result you come up with will be mass, or weight, of the object that is falling.