Answer
The distance between the kites is approximately $401$ft
Work Step by Step
To find the distance (which we'll assign the variable $x$), the law of cosines should be used:
$x^2=380^2+420^2-2(380)(420)\cdot $cos$(30º)$
$x^2=144,400+176,400-319,200\cdot 1/2$
$x^2=320,800-159,600$
$x=\sqrt{161,200}$
$x\approx401$ft
