$$
x+y=9;\ 0.5x+0.2y=3.75\\
0.5x+0.2(9.0-x)=3.75\\
0.5x+1.8-0.2x=3.75\\
0.3x+1.8=3.75\\
0.3x=1.95\\
x=6.5\\
y=9-x\\
y=2.5
$$

$$
x=\frac{-b\pm\sqrt{b^2-4ac}}{2a};\ 2x^2+15x-7.5=0\\
x=\frac{-15\pm\sqrt{15^2-4*2*-7.5}}{2*2}\\
x=\frac{-15\pm\sqrt{285}}{4}\\
$$