Correct spelling, explanation: when we want to create an abstract noun from an adjective, we add the suffix -th at the end. This way strong became strength as a noun. Strenght is incorrect because the suffix is supposed to be -th, not -ht. Another source for the correct spelling strength is the Old English form strengthu.

Definition of strenght:
noun, force, quality of having power
He didn’t have enough strength to move this container.
A lot of strength is important in athletic sports.

Idioms and phrases with strength
on the strength of, relying on:
On the strength of the Civil code, you are now married.

Incorrect spelling, explanation: strenght is not the correct form, because the suffix here is mistaken – it should be -th. Strength is therefore the correct form, as in Old English strengthu. Suffix -th with an adjective gives us the meaning of the abstract noun.

Incorrect spelling

Incorrect spelling

Incorrect spelling