Answer:
a) The slope = 0.144 , means that an increase in dissolved material by 1 mg/cm^2 will lead to an increase in Calcium content by 0.144 g/l 
while the coefficient of determination = 0.869 , means that 86% of the variation ( natural ) of calcium content is explainable using the Linear model.
b) 10.878 
c) S ≈ 1.99
Step-by-step explanation:
a) The slope = 0.144 , means that an increase in dissolved material by 1 mg/cm^2 will lead to an increase in Calcium content by 0.144 g/l 
while the coefficient of determination = 0.869 , means that 86% of the variation ( natural ) of calcium content is explainable using the Linear model.
<u>B) Point estimate of true average calcium content </u>
Given that dissolved material = 50 mg/cm^2 
= 3.678 + 0.144 * dissolved material 
=  3.678 + 7.2 =  10.878 
<u>C) Determine an estimate of error standard deviation </u>
Given that ; sum of squares ( SST ) = 320.398 
                    COD ( r )  = 0.86
first;  calculate the value of SSE = SST - SST ( r^2 ) 
SSE = 320.398 - 320.398 ( 0.86) ^2 
        = 83.43 
Finally ; determine the estimate of error standard deviation
S^2 = SSE / n -2 
       = 83.43 / 23 - 2 
∴ S ≈ 1.99