enthalpy change of combustion and formation help!!!!!!

i know what they mean but the exam questions confuse me because there is no website or resource to teach me how to answer a specific question on this topic.
when they give you the standard enthalpy of formation values for compounds and then they ask you to calculate the enthalpy of combustion? why dont you actually work the enthalpy of combustion like they ask in the question because in the markschemes it shows you do the reverse and use the opposite equation (products-reactants which is for enthalpy of formation). yet when I try this technique on another question which asks the exact style of question, the markscheme ignores the method above? it shows an equation for the combustion of a compound and shows the standard enthalpy of formation for the compounds involved in the equation and asks to work enthalpy change of combustion. the method where you use the opposite equation for the question asked and use the enthalpy of formation reaction, it is wrong?sorry it sounds so confusing but I have no idea either why is there nothing on this on any of the textbooks? they all just show how to work out just the enthalpy of combustion or formation separetly but not like in the format above where they include both.