First make all of your numbers that you are working with into the same unit. I choose mm. Then have this formula 18,900 squared = (180 * h) squared. Then take out the squared and you will have
18,900 = 180 * h. Now bring h to one side and everything else to the other side, this will look like this.
h = 18,900/180
The 180 is now dividing because it had been moved to the other side of the equation.
If you simplify it will look like this.
h = 105
So you have found the height of he back board.
To find the perimeter add all of the heights and lengths together, this will look like this.
(2*180) + (2*105) = 570
So the perimeter is equal to 579 mm