KOH + C4H4O4 ---> K2C4H2O4 + H2O
1) Balance K first as it is easier compare to O and H
2KOH + C4H4O4 ---> K2C4H2O4 + H2O
2) Now check for H and O, since K and C are already balanced. You need 2 more H and O on the right, logically, you will assign the 2 to H2O and not K2C4H2O4.
2KOH + C4H4O4 ---> K2C4H2O4 + 2H2O