What is the arithmetic mean of 8²,9²,10²,...,15²?

Let x - 3y + 4 = 0 and 2x - 7y + 8 = 0 be two lines of regression computed from some bivariate data. If b_yx and b_xy are regression coefficients of lines of regression of y on x and x on y respectively, then what is the value of b_xy + 7b_yx?

What is the mean deviation of the first 10 natural numbers?

Let $\rm \Sigma_{i=1}^9x_i^2=885$ If M is the mean and σ is the standard deviation of  x₁, x₂, x₃.....x₉ then what is the value of M² + σ²?