The -(- would be turned into a positive due to the rule of two negatives makes a positive. You wouldn't add the 4 into either of the numbers with a variable, you would only add the 10x and 7x.
Apply the distributive property to get 10x - -4-(7x). Now we can turn the two negatives into a positive to get 10x+4-7x. Now, subtract 7x from 10x to get 3x. This will leave you with 3x+4.
