His error is that he added the denominators x+3 and 2x to get 3x+3. When adding fractions, the denominators must be the same and they stay the same the entire time. We only add the numerators over this common denominator. Often, we'll want the lowest common denominator (LCD).
For example, if we add 1/8 and 2/8, then we get 1/8+2/8 = (1+2)/8 = 3/8. Think of it like slicing up a cake into 8 slices. Person A takes 1 slice while person B takes 2 slices. Overall, 1+2 = 3 slices were taken out of 8, so that's one way to see how 1/8 + 2/8 = 3/8.
If we added something like 1/4 + 1/8, then we cannot say the answer is 2/12. It's like adding apples and oranges. They aren't like terms in a way. But we can convert 1/4 to 2/8 since the fractions are equivalent
1/4 + 1/8 = 2/8 + 1/8 = 3/8
Those two one-eighth slices combine to get a quarter slice. In this example, 8 is the LCD.
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To fix his error, Jake needs to find the LCD of the denominators x+3 and 2x. The LCD in this case is 2x(x+3). He'll need to multiply top and bottom of the first fraction by 2x, as this is the missing factor needed to get to the LCD.
The missing factor for the second fraction is (x+3). So he'll need to multiply top and bottom of the second fraction by (x+3). After these steps are done, he can then add the fractions.