Beginner circuit diagram of superposition
You wrote: 


  2amps*(10ohms)/(10+25)ohms-3.5/25=.666


The right-hand side is correct (\$\dfrac{3.5}{25}\$) but the left one (\$\dfrac{2\cdot10}{10+25}\$) is incorrect, since it should be (\$\dfrac{2\cdot10}{10+15}\$), which does get you to .66 A.