In order to complete the square in the quadratic expression below, what will be the value of c? x^2-8x + c

we use a variable to represent any unknown value.   then we look for key words to tell us what operations have been performed.   for example, "sum" indicates addition, "product" is multiplication, "difference" is subtraction and "quotient" is division.   "is" means equals to.   we pay careful attention to the order in which things are listed in the problem, as we want to write them in the same order algebraically.