Evaluate the given limits, showing all working:
Using first principles (i.e. the method used in Example 1, Washington 2009, Using definition to find derivative) find the derivative of each function and then evaluate the slope of the tangent line at the given point.
a) y= 12x^{2} -x^{3} ; (1, 11)
b) y =x/x-1 (-3, 3/4)