做主 zuòzhǔ to take charge; to make the decision

怎么记住“做主”: 做 do + 主 lord = to take charge; to make the decision.

汉字

To make the decision / have the final say on a matter - 这事我做不了主 (zhè shì wǒ zuò bu liǎo zhǔ) I'm not in a position to decide this.
The pattern 替/为...做主 means to stand up and decide for someone (or right a wrong on their behalf): 你要为我做主 (you must take my side and set things right).
Also 当家做主 (dāngjiā zuòzhǔ) - to be master of one's own affairs. Colloquial, about authority to decide.