With simple regression, if we want to know the size of the effect, what is the best statistic to look at? And why?

With simple regression, if we want to know the size of the effect, what is the best statistic to look at? And why?

With regard to positive signs and negative signs, what is the relationship between the correlation coefficient and slope? And why?

If b = 3, the mean of X is 5 and the mean of Y is 10, what is the value of the constant? If you can please show me the math, that would be helpful.