That's the same approach I took, up until the Excel step
Restating as .98C - 1.99D = .5, you can see that C needs to be about twice as big as D
You can also see that every time you add 2 to C and 1 to D, you decrease the value on the left by 0.03.
Do that sixteen times as you get
.98*32 -1.99*16 = -0.48
Add one more to C, and you get .98-.48 = .50.