7(3x+4)=8(2x+5)+13

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=
28=-5+40+13
28+13=41
x=41

7(3x+4)=8(2x+5)+13

21x+28=16x+40+13

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.