We know the stuff inside the absolute value must be equal to 3.5 or -3.5. This is by the definition of absolute value.
Then 7/3x+0.3 = 3.5 and 7/3x+0.3 = -3.5.
Solving the first equation we get
7/3x = 3.2
x = 48/35
Solving the second equation we get
7/3x = -3.8
x = -57/35
