What does 3x plus x equal?

3x+x=4x. The 3x is just an easier way of stating x+x+x, and then you would just count the x's up like normal to create a total. You can also do this with larger numbers. For example, if you have 100x+500x, you would still add the numbers together to create a total of 600x. As long as the variables (x, in this case) are the same in the numbers you're adding (i.e., you can't add 3x+y together) you can just add the numbers to create your total.