Math question need help fast. ?

The lengths of the sides of a triangle are in the extended ratio 3:10:12 the perimeter is 400cm. What is the length of the longest side in cm. 2) Irene places a mirror on the ground 24ft from the base of an oak tree. She walks backward until she can see the top of the tree in the middle of the mirror. At that point, irene's eyes are 5.5ft above the ground,and her feet are 4ft from the mirror. How tall is the oak tree? Explain.