The five-carbon sugar in rna is called

Ribose sugar

Explanation:

This sugar is differentiated from deoxyribose sugar of DNA by the 2’ carbon of the carbon ring. This carbon in ribose has a hydroxyl group while that of deoxyribose sugar does  not. When this ribose sugar is bonded with a nitrogenous base on the 1’ carbon and a phosphate on the 5' carbon, it becomes a ribonucleoside that is the basic unit of RNA.

try using substitution:

x + y - 6 = x - y. (i'm not giving you the exact answer). since 0 = x - y, then sub 0 in the equation with x - y. combine your like terms, and then use inverse operations, and you'll have your answer.

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allowed for the prediction of the properties of missing elements.