The helicopter rose vertically, and then flew horizontally, so it's final displacement (distance from where it is now to where it started) is in the shape of a triangle.
The displacement is the hypotenuse of this triangle, with sides 300m and 400m.
We can work out this distance using Pythagorus' theorem, because we know that;
[tex] a^{2} = b^{2} + c^{2}[/tex]
Where [tex]a[/tex] is the hypotenuse, and [tex]b[/tex] and [tex]c[/tex] are the two other sides. So;
[tex] a^{2} = b^{2} + c^{2} \\ \\ a^{2} = 300^{2} + 400^{2} \\ \\ a^{2} = 90000 + 160000 \\ \\ a = \sqrt{250000} \\ \\ a = \boxed{500m}[/tex]
