The correct answer is letter B
        
             
        
        
        
Start here:  A = 18 ft^2 = L * W.  Next, L = 2W.
Merging these 2 formulas lets us eliminate L:
18 ft^2 = (2W) * (W) = 2W^2.
So W^2 = 9 ft^2, and W=3 ft.  Please use L = 2W (from above) to calculate L.
        
             
        
        
        
If there is such a scalar function <em>f</em>, then



Integrate both sides of the first equation with respect to <em>x</em> :

Differentiate both sides with respect to <em>y</em> :


Integrate both sides with respect to <em>y</em> :

Plug this into the equation above with <em>f</em> , then differentiate both sides with respect to <em>z</em> :



Integrate both sides with respect to <em>z</em> :

So we end up with
