You have the function
Notice that the first part of this hybrid function is a quadratic, and the second part is a linear function. Both are polynomials, and so are continuous everywhere. Therefore, the ONLY point where the function may be discontinuous is when you have to go from one of these part functions to the next, so in this case, where .
In order for the function to be continuous there, it needs to be defined there (which it is), and it also needs to approach the same value from the left as from the right.
So what value will the function approach if you are approaching from the left?
What value will the function approach if you approaching from the right?
If these values have to be the same, what does that mean c has to be?