Hi
Do you want projectile moving left and right like a guided missile or just slingshot and projectile should follow gravity and fall in arc shape line?
If natural slingshot projectile, you have it in FPS example blueprint. Bullets here moving on arch, they are projectile. All what you need is to set up projectile speed and weight - in short words. Lighter and faster projectile will move in more flat trajectory, where slower and heavier will be move more on arch trajectory following gravity. This all is done in engine and you not need any math for it.
Later you can just replace example gun to slingshot model and yellow ball/bullet to rock for example.