0.9÷(2.06-x)=4.5
4.5×(2.06-x)=0.9
4.5×2.06-4.5×x=0.9
9.27-4.5x=0.9
4.5x=9.27-0.9
4.5x=8.37
x=8.37÷4.5
x=1.86