In the mitochondria fatty acid elongation occurs by a reversal of beta-oxidation.
This shows the overall reaction of fatty acid elongation in mitochondria. The process is essentially a reversal of beta-oxidation, except that one NADPH and one NADH are required (beta-oxidation yields two NADH). Mitochondrial fatty acid elongation acts primarily on fatty acyl CoA substrates shorter than 16 carbons.
Notice that here acetyl CoA is the source of the added carbons.