So basically the perimeter of a parallelogram is
2*length + 2*width
Or, in this case, once you substitute everything in,
2 * (3x + 9) + 2 * (4x - 5)
Use the distributive property, to distribute the 2s to everything inside the parenthesis.
2 * 3x + 2 * 9 + 2 * 4x + 2 * -5
6x + 18 + 8x - 10
Then just add everything
14x + 8 = perimeter
