Well x isn't dependent on delta x in any way. So this is just going to be equal to x. What's the limit of x plus delta x as delta x approaches 0? Well as delta x approaches 0, this is just going to be equal to x. So if this approaches x as delta x approaches 0, and it's less than this function, and if this approaches x as delta x approaches 0, and it's always greater than this, then we know from the squeeze theorem or the sandwich theorem that the limit as delta x approaches 0 of c as a function of delta x is going to be equal to x as well. It has to approach the same thing that that and that is. It's sandwiched in between.