first off, let's recall that supplementary angles are just two sibling angles that add up to 180°.
so we have ∡T and ∡S, but we also know that ∡T = 3∡S, namely T = 3S.
[tex]\bf T+S=180\implies \stackrel{T}{3S}+S=180\implies 4S=180\implies S=\cfrac{180}{4}\implies S=45 \\\\\\ T=3S\implies T=3(45)\implies T=135[/tex]
