The formula you use is a^2 + b^2 = c^2
Let's set up the equation 30^2 + 42^2 = c^2
a and b are ALWAYS the legs of the triangle. c is ALWAYS the hypotenuse.
Since you're finding c, the shadow's length, then we leave that as c instead of plugging in a number.
Next, find the square of 30 and 42, to leave the equation:
900 + 1,764 = x^2
add 900 + 1,764 to make 2,664.
2,664 = x^2 is what we're left with.
All you need to do now is square root 2,664 to find x instead of x^2
x = 51.614
Your answer is the last one, 51.6 feet.