Expand the left hand side:
[tex] 3x+0.3 = 1.3 [/tex]
Subtract 0.3 from both sides:
[tex] 3x = 1 [/tex]
Divide both sides by 3:
[tex] x=\dfrac{1}{3} [/tex]
Note that 0.33 is just an approximation. In fact, if you substitute that value for x in the equation, you get
[tex] 3(0.33+0.1) = 3\cdot 0.43 = 1.29 \neq 1.3 [/tex]
Instead, if you use the exact value, you have
[tex] 3\left(\dfrac{1}{3}+0.1\right) = 1+0.3 = 1.3 [/tex]
