Operations are performed according to the Order of Operations. Sometimes the mnemonic PEMDAS or BIDMAS is used to remind you what the order is.
P/B - parentheses/brackets. The content of these is evaluated first.
E/I - exponents/indices. Exponentiation is done first, right to left: a^b^c = a^(b^c).
MD/DM - multiplication and division are done in order of appearance, left to right. Each has equal priority, neither is done before the other unless it appears in the expression first. a/bc = (a/b)c. ab/c = (ab)/c
AS - addition and subtraction are done in order of appearance, left to right. Each has equal priority. _____ When functions are involved (sin( ), log( ), sqrt( ), for example), their arguments are evaluated according to the order of operations, then the function is evaluated, then the remainder of the operations are performed. For example, sin(a)^2 = (sin(a))^2. Sometimes, this is written sin^2(a).
When functions are written without parentheses around their arguments, it must be assumed that the function only applies to the first entity following the function name. log ab+c/d = (log(a)*b)+(c/d), for example, or √3x = (√3)x.
The third one down Because the best line of fit is a line that goes between the plotted points and it HAS to be a straight line and most of the points close to the line