This is correct, except that if you have two variables you will need two equations. The other equation (the easy one!) was x+ y = 100, from which you might want to solve for y = 100-x, and substitute into the other equation to get:
.30x + .02(100-x) = .04(100)
.30x + 2 - .02x = 4
.28x = 2
x = 2/.28 = 50/7 or approximately 7.14 gallons of cream
100-x = 92 6/7 or approximately 92.86 gallons of 2%
