TO find perimeter you add all sides. But to find the totaly of each side, you will divide. So a triangle has 3 sides, so you divide 45 by 3 to get the sides length of: 15 cm each ~hope this helped :)
The line through that point and the origin has a slope of -1. It is perpendicular to the line you want, so the line you want has slope +1. Its equation can be written as .. y = (x -4) -4 or, in standard form .. x - y = 8