Question 175607
You need to use the pythagorean theorem.  It states that for a right triangle the the length of both of the legs(a and b) squared is equal to the square of the hypotenuse(c).  or in simple language a^2 + b^2 = c^2.

Now we know one of the legs is 30 meters(the distance from the car to the track).  The hypotenuse is 50 meters(distance from the train to the car).  Now we just need to solve for the other leg.

a^2 + b^2 = c^2
(30)^2 + b^2 = (50)^2
b^2 = (50)^2 - (30)^2 = 2500-900 = 1600 meters

now to find the leg we need to take the square root of 1600 which is 40.  So your final answer {{{highlight(b=40)}}}meters