If W=width then the length is 43+W and the area is W(43+W)=4320.
So, 43W+W²=4320, giving us a quadratic: W²+43W-4320=0.
This can be solved using the quadratic formula:
(-43+√43²+4×4320)/2=(-43+√19129)/2=47.654 feet approx. The other root of the quadratic is negative so it is meaningless in this case, because measurements can’t be negative feet. The length is 43 feet longer: 90.654 ft approx.
(Incidentally, if the length had been 42 feet longer than the width, the answer would have been exactly 48 ft for the width and exactly 90 ft for the length.)