Answer: 
Step-by-step explanation:
The confidence interval for population mean is given by :-

Given : Sample size :
, which is a large sample , so we apply z-test .
Sample mean : 
Standard deviation : 
Significance level : 
Critical value : 
Now, a confidence interval at the 95% level of confidence will be :-

X=12.5
If the perimeter is 90
90=3x+3x+15
Combine like terms
90=6x+15
Subtracts 15 from both sides
75=6x
Divide by six to get x by itself
12.5=x
Does this help?
The answer is nine: 17 minus 8 is 9
The constant is 3 because it has no variable.
The coefficient is 7b because it has a variable.