You would need to create an equation for this problem. Let x represent peter's money.
(2 · x) + $3 = $10
Now, all you would have to do is solve it.
First, you would need to simplify it.
2x + $3 = $10
Then, you would need to combine like terms.
2x = $7
The last step would be to isolate the x. To do this, you would divide both sides by 2.
x = $3.5
Peter has $3.5.
I hope this helps!