You have to use the Pythagoras Theorem (a² + b² = c²) twice to find the length of just one slant of wood. By finding one slant of wood, you can multiply the length by four (the number of slants) and then add sixty (the length of the wood adjoining the four slants.
First draw a box with the dimensions of 120x120x60 cm, as with those dimensions, the slant fits into the box perfectly with both sides of the slant fitting in two opposite corners. Start with 120² + 120². You don't have to square root it to get "c", as we will have to square it to get the final answer. Finding the square root of it then squaring it will make nothing happen .120² + 120² is 28800. That is "a²" for the second time you use Pythagoras Theorem. 28800 (or a²) + 60² (or b² or 3600) = 32400, which is "c²". The square root of 32400 is 180, which is "c" and is the slant.
180x4 will get us all the slants. It is 720. 720 + 60 = 780cm, which is the length of all the wood.
To find if the length of the wood can be minimised by changing the red line, just do all the steps again, and you will find out that the amount of wood will decrease by changing the red line.