well, the triangle is an isosceles, so it twin sides, and the twin sides make twin angles, as we see by the tickmarks on A and C, meaning AB = BC.
![\bf x+4=3x-8\implies 4=2x-8\implies 12=2x\implies \cfrac{12}{2}=x\implies 6=x \\\\[-0.35em] \rule{34em}{0.25pt}\\\\ AC\implies x+4\implies 6+4\implies 10](https://tex.z-dn.net/?f=%5Cbf%20x%2B4%3D3x-8%5Cimplies%204%3D2x-8%5Cimplies%2012%3D2x%5Cimplies%20%5Ccfrac%7B12%7D%7B2%7D%3Dx%5Cimplies%206%3Dx%20%5C%5C%5C%5C%5B-0.35em%5D%20%5Crule%7B34em%7D%7B0.25pt%7D%5C%5C%5C%5C%20AC%5Cimplies%20x%2B4%5Cimplies%206%2B4%5Cimplies%2010)
3/4 (x-12)=12
3/4x - 9= 12
+9 +9
3/4x=21
*3/4 *3/4
x= 15 3/4
and
3/4y-12=12
+12 +12
3/4y = 24
*3/4 *3/4
y= 18
A supplementary angle is an angle in which two of it's measurements can be added to equal 180 degrees.
For example, a measure of 130 and a measure of 50 in the same shape.
They can both be added to equal 180.