Genesis 4:15, 24

And the LORD said to him, “Therefore, whoever kills Cain, a sevenfold vengeance [that is, punishment seven times worse] shall be taken on him [by Me].” And the LORD set a [protective] mark (sign) on Cain, so that no one who found (met) him would kill him. [Gen 4:24]

“If Cain is avenged sevenfold [as the LORD said he would be], Then Lamech [will be avenged] seventy-sevenfold.”