To find the perimeter of a figure, you just need to add all sides together. First, you need to set up an equation that adds all sides together. Then, you would need to combine like terms (x+x+x=3x); -3+6=3. So you would take 3x and add 3, since 3 is positive.
knowing that the system are y=-x+6 and y=x+2, then: x+2=-x+6; x+x=6-2; 2x=4; x=2, and replacing x in y we have: y=-2+6=4 and y=2+2=4, finally tha solution to sistem is (2,4)