There is 1 serving of lemon juice to 3 cups water. Hence why it's 1:3 ratio.
Since it's 1:3, then 2 servings to 6 cups of water, so on and so forth.
Multiply your lemon juice servings by 3 to find out how many cups of water for the table.
To find how much lemon juice for 36 cups of water, just divide 36 by 3 to get 12. 12 servings of lemon juice is just right for your 36 cups of water.
So.. take a peek at the picture... let's get two points from it, hmm say 0,4 notice it touches the y-axis there, and say hmmm -4, 1, almost at the bottom of the line
once you get the slope and solve for "y", that'd be the equation of the line.