Need total kills scoreboard

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I'm trying to create a scoreboard sidebar for a map I'm making and it needs to be the total amount of zombies killed. I also want it to add 10 points for each zombie killed and display when the player presses tab. Unfortunately I know nothing about scoreboard and can't seem to find the answer anywhere else. Please help!
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How do you add a kill count to scoreboard?
Via the /scoreboard objectives setdisplay command (see command reference), players' scores in specific objectives can be displayed in certain 'slots' in-game. Each 'display slot' can show one objective at a time, and multiple 'display slots' may be used for the same or different objectives.Player Kill Scoreboard in Minecraft tutorial
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Images: Anna Tarazevich, Karolina Grabowska, Julia Volk, Julia Volk