The proof is because it has a right angle.... I think :)
If a line is parallel to another line,Then the gradients are the same.So m=4 .Using general formula for str8 line, y=mx+c

sub in coorfinates (1,13)

So C=9.
We know, opposite sides are parallelogram is parallel, therefore slopes are equal.
Slope of line one :

So, equation of other line passing through (-4,5) and have slope of -0.5 is :
y-5 = -0.5( x-(-4))
y-5 = -0.5x - 2
y + 1/2x = 3
2y + x = 6
Therefore, equation of line containing the (-4, 5) is 2y + x = 6.
Hence, this is the required solution.