Which is the best way to combine sentence The tone of speech was sternThe speaker called for hard work and determination?

The best way to combine these sentences depends on the desired emphasis. Here are a few options:

Emphasis on the tone:

* The speaker's stern tone demanded hard work and determination. (This emphasizes the demanding nature of the speaker.)

Emphasis on the call to action:

* The speaker, with a stern tone, called for hard work and determination. (This emphasizes the speaker's call for action.)

More concise option:

* The speaker, in a stern tone, demanded hard work and determination. (This combines the two sentences concisely and clearly.)

Ultimately, the best way to combine the sentences depends on the specific context and the desired effect.