Challenging Statistical Significant

In 18 patients with Everley's syndrome, the mean level of plasma phosphate was 1.7 mmol/l (millimoles per liter) with a standard deviation of 0.8 mmol/l. If the mean level in the general population is 1.2 mmol/l, is there a statistically significant difference between the general population mean and these 18 patients' mean? Make sure to go through all the steps for hypothesis testing: state the appropriate test to use, state the hypotheses, and then do the calculations and providing a brief explanation of your results. Conduct a two-sided test.

My Work/Equations/test
1)Test to use: One sample t-test. 2)Hypothesis:There is no significant difference between the general population and the sample size.
a)Steps for Hypothesis testing-the data are quantitative and plausibly Normal, the two samples come from distributions that may differ in their mean value, but not in the standard deviation, the observations are independent of each other. Null hypothesis is that the two groups come from the same population.
b)Two sided test H0: = k ;Ha: k.; Appropriate significance test is the z-test. Mean of gen population = 1.2mmol/1; Mean of sample x=1.7mmol/1;SD sample=0.8mmol;SEM=SD/sqt(n)=0.8/sqt(18)=0.8/4.24=0.189mmol/1;Difference between means -x=1.2-1.7= -0.5; t=difference between means divided by standard error of sample. -0.5/0.189= -2.64 comes between the probability values of 0.02 and 0.01(2% and 1%), so it is unlikely that the sample mean of 1.7 came from the population mean of 1.2. The sample mean is statistically unusually high.
Is the negative sign important?
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