To multiply a reaction by a factor in a Hess law problem, you need to multiply all the coefficients in the reaction by that factor, including any coefficients for the enthalpy change.

Let's say we have the following reaction:

$$A + B → C + D \ \ \ \ \ \ \ ΔH = -100 kJ$$

If we want to multiply this reaction by 2, we would get:

$$2A + 2B → 2C + 2D \ \ \ \ \ \ ΔH = -200 kJ$$

So the enthalpy change for the multiplied reaction is simply the original enthalpy change multiplied by the factor we used to multiply the reaction.