suppose that f(5)=5 and
the limit as x->5 is 4f(x)-20/(x-5)(x+5)=10
find derivative of the function.
as x-->5, top and bottom both --> 0 so use l'hopital
top d/dx (4y-20) = 4 dy/dx
bottom d/dx (x^2-25) = 2x
so (4 dy/dx) /2x = 10 as x--->5
2 dy/dx = 10*5 = 50
dy/dx = 25