Write an equation that expresses the fact that a function f is continuous at the number 4.

  Use the definition of continuity. 
A function is continuous at a point x = a iff 
lim [x → a] f(x) = f(a) 

In other words, a function is continuous at a point if its value is equal to its limit at that point. 

So, for f(x) to continuous at x = 4 we must have 
lim f(x) = f(4) 
x → 4 

Y=-2x-6 is the answer