3a+4a=14 well we are combining terms so 3a and 4a will be 7a=14. Now we have an equality so we isolate the variable to give a the answer of 2. Then for b+6b=21 we do the same and combine terms having 7b since the single b there will always be a 1. Then we isolate the variable Again getting 3 for a. Lastly for 7(a+b) you have to disturb a and b with 7 which means (7*2)+(7*3)=14+21=35