So we know that the formula for the area of a rectangle is \(A = lw\).
Now both the length and width of the rectangle increase at 3 km/s, therefore, \(A(t) = (3t+l)*(3t+w). Since the initial length = initial width = 4 km, then the initial area = 16 [tex]km^2\). We want to know the time when the area is four times its original area, therefore, our new formula is: \(4A(t) = (3t+l)*(3t+w)\). Plugging in our known values we have:
\(64 [km^2] = (3t + 4 [km])*(3t + 4 [km])\)
\(t = \frac{4}{3} s\)
The area is four times its original area after \frac{4}{3} s[/tex].