Question 243097
You can use the method of 'like ratios'.   We are told that the ratio of water to polenta is 2 to 1/2, which can be shown as:  2/.5

We are told that we're interested in how much for 3/4 c. of polenta.  So that ratio will be x/.75.

The ratios have to be equal, so we need to solve:  
2/.5 = x/.75

Cross multiplying, we arrive at
2(.75) = x(.5)
1.5 = .5x
Dividing by .5
1.5/.5 = x
x = 1.5/.5
x = 3