I've managed to find a solution to the following question using the f(a+h) - f(a) / h definition of the tangent line limit, but I got caught up trying to get the same answer using the: lim x -> 1 f(x) - f(1) / x-1 definition:

If a rock is thrown upward on the planet Mars with a velocity of 10 m/s, its height (in meters) after t seconds is given by H = 10t - 1.86t^2

(a) find the velocity of the rock after one second (I got 6.28 m/s)

(b) find the velocity of the rock when t = a (I got 10-3.72a)

The question doesn't ask about the second definition but I'm just curious to know what mistake I made while trying to calculate it using the f(x) - f(1) definition. Thanks for any clarification