Verified answer

Step One:

Add Water to the side with less oxygens:

Sn(OH)4^2- + 2H2O --> Sn(OH)6^2-

Step Two:

Add H+ to the side that you didn't add the waters to to balance out the remaining hydrogens:

Sn(OH)4^2- + 2H2O --> Sn(OH)6^2- + 2H+

Step Three:

Add electrons to balance out the charge:

Sn(OH)4^2- + H2O + 2e- --> Sn(OH)6^2- + 2H+

Step Four:

Make sure everything else is balanced, which it is.

b. Bi(OH)3 + H+ --> Bi + 3H2O + 3e-