The largest area that can be enclosed will be that of a square (140 ft)/4 = 35 ft on a side. Such a square has an area of (35 ft)² = 1225 ft², not enough for the garden to be 1300 ft². This tells us the solution to the equation you're being asked to write is non-real.
_____
Putting the given numbers in the area equation, you have
... 1300 = 70w -w²
Multiplying by -1 and completing the square, we have
... w² -70w +35² = -1300 +35²
... (w -35)² = 1225 -1300 = -75
Taking the square root shows us the solution is non-real.
... w - 35 = ±√(-75) = ±5i√3
... w = 35 ± 5i√3 . . . . . . . . . after adding 35. These are complex solutions.
