[tex]12 - 7.8x = 8.412 \\ 7.8x = 12 - 8.412 \\ 7.8x = 3.588 \\ x = 3.588 \div 7.8 \\ x = 35.88 \div 78 \\ x = 0.46[/tex]

[tex]1.8x + 2.4x - 0.8 = 0.46 \\ 4.2x - 0.8 = 0.46 \\ 4.2x = 0.46 + 0.8 \\ 4.2x = 1.26 \\ x = 1.26 \div 4.2 \\ x = 12.6 \div 42 \\ x = 0.3[/tex]