Berean Standard Bible (BSB)
being fully persuaded that God was able to do what He had promised.
World English Bible (WEB)
and being fully assured that what he had promised, he was also able to perform.
Strong's King James
And{G2532} being fully persuaded{G4135}{(G5685)} that{G3754}, what{G3739} he had promised{G1861}{(G5766)}, he was{G2076}{(G5748)} able{G1415} also{G2532} to perform{G4160}{(G5658)}.
The F.O.G *
(21) Completely convinced that what was promise-vowed, He was powerful enough to do.
The F.O.G Message *
Young's Literal Translation
and having been fully persuaded that what He hath promised He is able also to do: