Firstly, you should think of what would react to form Zinc chloride and what Zinc chloride even is.Looking at the periodic table and with knowledge from class, I as a student should know that this is an example of a metal salt.
I would also need to think of the formula of the salt - i.e. from it's position in the periodic table and it's electron configuration I can find that it has a charge of 2+ as an ion, and chlorine has a charge of -1 as an ion. This means the product of zinc chloride would be ZnCl2.
This would mean that in order to form it, there would be the reaction of zinc with hydrochloric acid.This would give me the foundations to start writing an equation; I.e.Zn + HCl --> ZnCl2 + ??
At this stage looking at the reactants I can see that there is hydrogen left over and this would be used to form hydrogen gasZn + HCl --> ZnCl2 + H2
The next stage is stoichometry and balancing out the equation;This would giveZn + 2HCl --> ZnCl2 + H2
Finally, we look at the states of the reactants and products and add this to the equation to ensure all marks are picked up in the question
Zn (s) + 2HCl(aq) --> ZnCl2 (aq) + H2 (g)