when isaac Newton was inventing calculus, how did he do it? why do we get the antiderivative? why do we have to F(b)-F(a)? I am just interested to know.
I really have no clue what you are asking. Have you taken a Calculus course? Why we would want to find an anti-derivative and why we subtract the values of the anti-derivative at two different points are explained in any Calculus text.
And they have nothing to do with how Newton (and Leibniz) developed Calculus to begin with.