I think they are talking about the formula for the right triangle: a^2+b^2=c^2, and they gave you the height of a=18, and the width of b=4, so c would be the length of the hypotenuse. 18^2 + 4^2= c^2
324+16=c^2
340=c^2
Square root of 340=c
Square root of 340 can be factored by pulling a 4 out from under the square root.
2(square root of 85)=c
