Take 3, put parentheses around it like this

(3)

and then write 6y-5, but substitute (3) in place
of y, like this

6(3)-5

Multiply 6 times (3) getting 18. 

Substitute 18 for the 6(3) and get

18-5

Then subtract 18-5, and get

13.

That's the answer.

Edwin