Answer:
20,800
Step-by-step explanation:
Let's take a look at a perfect square trinomial.

We can distribute this expression to get

.
An important relation to recognize here is that the constant is the square of half of the x value.
Now, if we wanted to add a number to

to make a perfect square trinomial, we could have the value of x and then square it.
Half of 16 is 8, and 8 squared is
So -5x=-50
multiply both sides by -1
5x=50
divide by 5
x=10
check
-5 times 10=-50
-50=-50
check