Please help I don't get it

Variables a and b represent the lengths of the legs of a right triangle, and c represents the length of the hypotenuse. Express your answer in simplest radical form

Find c if a=4 centimeters and b=8 centimeters

c=_________cm

2. A diagonal of a square parking lot is 45 meters. Find to the nearest meter, the length of the side of the lot

__________m

3. Simplify and reduce expression

-12+/- radical 216 / 6

So it is negative 12 plus or minus radical 216 all over 6

Someone please help me understand this thanks