Question

4.9a+1.74=4.03a

Answer 1

how do you know which way to add up or subtract

Answer 2

Put all the terms with a variable, in this case "a", on one side, and all the numbers without a variable on the other side.

To start you off...
4.9a-4.9a+1.74=4.03a-4.9a

Answer 3

does it matter what side?

Answer 4

Nope. I like to try to keep everything positive, but in this particular situation, there's no way to do that. You're going to end up with a negative on one side.