“Correlation does not imply causation.” Repeat that until it hurts.
You don’t need to calculate standard deviations by hand. Computers do that. Your job is to think. Statistics For Dummies
Minimize sum of squared residuals: $$\hat\beta_1 = \frac\sum (x_i - \barx)(y_i - \bary)\sum (x_i - \barx)^2$$ “Correlation does not imply causation