Prove using alphabets how triangle acd is proportional to triangle aef (f is on the right side)

Given the following sampling distribution of one mean with a sample size 49, from a normally distributed population, find the population standard deviation, o.79828588912497

What is the standard deviation of the following data set rounded to the nearest tenth? 3, 17, 18, 15, 12, 21, 9

For what values of x is x2 + 2x = 24 true?