Answer: yes when your adding or subtracting fractions you must find the common denominator between the two before you continue. 
Step-by-step explanation:
Example: 2/4+3/2 
Find the common denominator. ( in this case the common denominator is 4) 
We keep the first fraction as it is cause it has the denominator 4 and we multiply the second fraction by 2 up and down to get a denominator of 4. The equation becomes 2/4+6/4
Now you add the numerators and you got your answer ( don’t forget to simplify) 
The answer after adding in this case is 8/4 and when simplified it is 2 
 
        
             
        
        
        
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Note :- If sample size n > 30 OR Population standard deviation σ is given then We use Z test.
 If sample size n < 30 AND Population standard deviation σ is unknown then we use t test.
In this question we are not given Population standard deviations σ1 and σ2.
Also n1 = 14 and n2 = 13 both sample sizes are LESS than 30.
Therefore we use t test.
Given :- Assume the population standard deviations are equal. ( σ1 = σ2)
then Degrees of freedom = n1 + n2 - 2 = 14 + 13 - 2 = 25.
(kindly find the image attached with this solution)
Answer :- Therefore 25 degrees of freedom are used to find the t critical value.
 
        
             
        
        
        
Try plugging in 4,6, and -7 into the x's