4.2x + 8.4 = 14.7
Subtract 8.4 from both sides:
4.2x = 6.3
Divide by 4.2:
x = 1.5

0.39x - 0.1 = 0.16
Add 0.1 to both sides:
0.39x = 0.26
Divide by 0.39:
x = 0.666667 or approximately 0.67