Assume that the number is x
The opposite of 3 is -3
Since the sum of the number and the opposite of 3 equals 4
Therefore:

Remember (+)(-) = (-), so

To find x we need to move 3 to the other side by adding 3 to both sides

Since x represents the number, so
The number is 7
Let us check the answer
7 + -3 = 4
The answer is correct