What I believe you meant to ask is, "Which expression equals 6 when a=5 and b=1/3?". If that's the case...
What they are really asking is, "How can we get "a+b" to equal 6?"
What values must a and b be to make this statement true?
If we plug a=5 (given) into the equation, we get:
5+b=6
Solve...
b=1
b must equal 1 for "a+b" to equal 6
b=1/3 (given)
What can we do to 1/3 to make it equal 1?
Multiply it by 3!
5+((1/3)*3)=6
Putting a and b back into the equation, we get:
a+3b=6
