An algebra tile configuration. Only the Product spot is shown. 15 tiles are in the Product spot in 6 columns with 6 rows. First row: 1 + x squared, 5 negative x. Below the + x squared tile are 5 negative x tiles. Off to the right are 4 negative tiles in a separate column.
How many zero pairs must be added to the function
f(x) = x2 – 10x – 4 in order to begin writing the function in vertex form?

4
10
21
25