Answer
$1250 \ ft$
Work Step by Step
Create a proportion using ratios as follows:
$$
\frac{750\ ft}{1.89\ \Omega }=\frac{x\ ft}{3.15\ \Omega}
.$$
Set the product of the means equal to the product of the extremes and solve for $x$
$$
1.89 x=750\cdot 3.15\Longrightarrow x=\frac{750\cdot 3.15}{1.89}=1250 \ ft
.$$
