supplementary angles sum up to 180°, so let's say we have two angles, a and b, a + b = 180, but we know that a is twice the measure of her partner, so a = 2b.
[tex] \bf a+b=180\implies \stackrel{a}{2b}+b=180\implies 3b=180\implies b=\cfrac{180}{3}
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\boxed{b=60}~\hspace{5em}a=2b\implies a=2(60)\implies \blacktriangleright a=120 \blacktriangleleft [/tex]
