If you have 6 moles of reactant a and excess of b and c, how much product e would be formed? 3a+5b+1c=> 2d+1e

From the reaction you can state the molar ratios:

3 mol A : 5 mol B : 1 mol C : 2 mol D : 1 mol E.

Given that B and C are in excess, you know that reactant A is the limiting reactant, which means that is reacts completely. So, you can use the number of moles of reactant B to find how many moles of the products are produced.

For that, you build a proportion with the molar ratios between A and E,  and the number of moles of A that reacted:

=> 3 mol A / 1 mol E = 6 mol A / x => x = 6 mol A * 1 mol E / 3 mol A

=> x = 6*1/3 mol E = 2 mol E.

2 moles of E.