Answer
$x=6$ cubits
Work Step by Step
Assume the length of the reed is $x$ cubits, we are given the following conditions:
1 ninda =12 cubits, so 1 squre nindas =$12^2$ square cubits,
area of the field=$375$ sqaure nindas=$375\times12^2$ sqaure cubits,
length of the field = $60\times(x-1)$,
width of the field =$30x$.
The above conditions give the equation $60(x-1)\times30x=375\times12^2$ which lead to
a quadratic equation $x^2-x=\frac{375\times12^2}{60\times30}=30$, or $x^2-x-30=0$
As $x$ must be positive, the only solution for the equation is $x=6$ cubits
