Question 1062309
a+b+c=10
a+b-c=3 
solve for c in each
c=10-a-b
c=a+b-3
Set those two equal to each other
a+b-3=10-a-b
2a+2b=13
a+b=6.5, dividing both sides by 2
now substitute that into both equations
6.5+c=10
6.5-c=3
Both will give c=3.5
Therefore, a+b+3c=6.5+3(3.5)=6.5+10.5=17