What is the Chemical equation of potassium dichromate when HCl is added to the solution?

In K2Cr2O7 there are 2 Cr atoms that are +6 each and in CrCl3 Cr is +3 and so you'll have 2 Cr atoms going from +6 to +3 which is a total of 6e- (Cr2O7^2- + 6e →2Cr3+ ); important to balance the Cr atoms here to calculate the number of e-).

In HCl the Cl atom is -1 and in Cl2 the 2 Cl atoms are in the zero oxidation state and so you'll have 2 Cl atoms going from -1 to 0 which is a total of 2e- (2Cl- → Cl2 + 2e- ); also important to balance the Cl atoms here to calculate the number of e-)

From here you should recognize that to balance the electrons you'll need to balance each half reaction to 6e- to make them equal so you'll have to triple the second half reaction.

After that you'll need to (1) balance the oxygens using H2O

and (2) balance the hydrogens using H+. ( Cr2O7^2- + 14H+ + 6e →2Cr3+ + 7H2O )

The last balanced equation will be

K2Cr2O7 + 14 HCl → 2 KCl + 2 CrCl3 + 7 H2O + 3 Cl2

Hope this helps!