Slope= -3/2
Y-intercept=33/2
First, let's define our variables.
Let's call our width X(given)
and our length, since it's 10 shorter than 3 times the width, we can call our length:
3x-10
So, the perimeter of a rectange is equal to 2 times the length + 2 times the width.
So, when we plug our values in, we get

The perimeter is 8x-20
Supplementary angles add up to 180 degrees. Using this, we can set up an equation.
Angle 1 + Angle 2=180
124+(2x+4)=180
2x+128=180
2x=52
x=26
The value of x is 26