Answer
$x = 63.0~m$
Work Step by Step
We can find the path length difference when the phase difference is $1.00~\lambda$:
$1.00~\lambda = (1.00)(2.00~m) = 2.00~m$
We can form a right angle triangle with sides of length $d$, $x$, and $x+2.00$
We can find $x$:
$x^2+d^2 = (x+2.00)^2$
$x^2+d^2 = x^2+4.00~x+4.00$
$d^2 = 4.00~x+4.00$
$4.00~x = d^2-4.00$
$x = \frac{d^2-4.00}{4.00}$
$x = \frac{(16.0)^2-4.00}{4.00}$
$x = 63.0~m$