posted by patti .
how would I start a equation like this would it be 21x+28=16x+40+13 ?
Yes, that is the way to start. Now collect the x's on one side.
so it would be 21x-21x=28=
To get all the x values on one side and the other numbers on the other side, subtract 16x and 28 from both sides.
I don't know how you got
That will leave you with a much different answer.
I hope this helps a little more. Thanks for asking.