Equivalence of definitions of upper contour sets

Mas-Collel, Winston and Green's Microeconomic Theory (3rd editions) offers two definitions of the upper contour set:

How can the equivalence between the two definitions be proved?

• But Def M.C.3 is not a definition of the upper contour set; rather, it's a definition of quasiconcavity of a function. – Herr K. Feb 26 '19 at 19:57
• Or are you asking about the equivalence between $\{x\in A:f(x)\ge t\}$ and $\{y\in X:y\succsim x\}$? – Herr K. Feb 26 '19 at 20:00
• I have updated the question. The definition of upper contour set in M.C.3 is now highlighted. – Bruno Schiavo Feb 26 '19 at 21:29
• Have you given the question some thoughts? This is not a particularly difficult proof. – Herr K. Feb 26 '19 at 21:33