Having trouble with a couple of questions

The height of a projectile at a particular time (measured in seconds) is given by h(t)=10t(1-t)m. Find the rate of change of height from first principles (using the definition of the rate of change).

The first one:

- How do I find the rate of change of height and I'm a bit confused on what 'first principles' are.

The second:Consider the function f(x)=x(4-x). Find the equation of the tangent to the graph of f(x) at x=2. [NOTE: when calculating f'(2), use first principles.]

- Again I'm not too sure on the 'first principles' thing.

Any help is much appreciated,

moob