chemistry question!

When making half equations, you can add hydrogen ions (H+) and water (H2O) to either side.

So, for the first one, firstly you can see that there are two chromium atoms on the left hand side and one on the right, so the Cr3+ becomes 2Cr3+.
Now you can see that the left hand side has 7 oxygen atoms, while the right hand side has none. Therefore you can add 7H2O to the right hand side.
Now there are 14 hydrogen atoms on the right hand side and none on the left, so add 14H+ to the left hand side.
The charge for the left hand side is now +12 (-2+14=+12) while the charge for the right hand side is +6, therefore to balance the charges on each side you add 6e- to the left hand side, making the charges both equal at +6.

For the second equation, again there are 4 oxygens on the left hand side and none on the right, so add 4H2O to the right hand side.
This gives 8 hydrogens on the right hand side, so add 8H+ to the left hand side.
The overall charge on the left hand side is now +6 (-2+8=6) while the charge on the right hand side is 0, so add 6e- to the left hand side to make the charges equal.


I hope that this helps!