you can solve this in two easy ways. First off we'll do it mathematically, using what's called the Pythagorean Theorem. This is as follows (only for right triangles!): a^2+b^2=c^2
in this equation, a and b are the two sides, or legs, of the triangle that form a right (90 degree) angle, and c is the other, diagonal side, aka the hypotenuse.
For your question, a and b (the two sides of the triangle) are 300 and 400m. using the theorem i put above,
300^2+400^2=c^2 (you want to find c, as that is how far the helicpoter is from the start)
250000=c^2
c=500m
and there's your answer
the other, much easier way is to know the fact that one example of a right triangle, aka a "Pythagorean triple", has sides of 3 and 4 and a hypotenuse of 5. Knowing that, one could solve your question in a matter of seconds.
so there you go. 500m
