A Level Maths Stats

Hi, I struggle with this style of question, as I'm not sure how to use the expressions when they aren't something familiar such as Σx^2 or Σx etc. I managed to work out the mean by thinking of the Σ(m-150) = -182 as (m1 - 150) + (m2 - 150) + ... = -182, so (m1 + m2 + ...) - 52(150) = -182 and then rearranging for m1 + m2 + ..., so Σx, and then dividing by n. In terms of the variance, I'm a bit puzzled and don't know how to use the Σ(m-150)^2. Thank you in advance