Ok so from the x^2 + 3x if you take out the common factor, you would be left with (x+3) and then the left over x. and from the 1x+3, if you take out the 1 you will just be left with (x+3). Since the (x+3) is the common factor of the two equations that will be one of the factors, and the leftover and 1 will form another factor of (x+1). Making the factor (x+3)(x+1). But there's also other ways of factoring if it's too confusing, what I do is find numbers that multiply to make 3. This would be 1 and 3 since 1x3=3. Since both of the parts in the polynomial is positive the factors would both be positive. To check to see if the numbers are going to be right you can add up the multiples, so 3 and 1, 3+1 would be 4. After you check that you can factor them (x+1)(x+3) the order doesn't matter. This is just the way that I do it.