Use elimination or substitution to solve.
I'll use substitution here because the second equation can easily be put in terms of one variable.
x+y=9.0
y=9.0-x
substitute 9.0-x into the other equation for y
0.5x + 0.2(9.0-x) = 3.15
0.5x + 1.8 - 0.2x = 3.15
0.3x + 1.8 = 3.15
0.3x = 3.15 - 1.8
0.3x = 1.35
x = 1.35/0.3
x = 4.5
then if x = 4.5
4.5 + y = 9.0
y = 9.0 - 4.5
y = 4.5
