Answer:
See the attachment for what goes on your X diagram.
Rewrite: x² -7x +4x -28
Grouping: (x² -7x) +(4x -28) = x(x -7) +4(x -7) = (x +4)(x -7)
Explanation:
The given quadratic is ...
... x² -3x -28 . . . . . a=1, b=-3, c=-28
a) The value at the top of the X diagram is the product a·c = 1·(-28) = -28.
The value at the bottom of the X diagram is the coefficient b = -3.
The values on the sides of the diagram are the factors of -28 that add up to make -3. These are -7 and 4. That is, ...
(-7)·(4) = -28
(-7)+(4) = -3
b) Since the two values on the sides of the diagram add up to give "b", the value of "b" in the equation can be rewritten as the sum of these two numbers. Doing that, we have ...
... x² -3x -28
... = x² +(-7+4)x -28
... = x² -7x +4x -28 . . . . . . order does not matter. It could also be x² +4x -7x -28
c) We can group pairs of terms in the rewritten expression and factor each pair.
... = (x² -7x) +(4x -28) . . . . . first pair has a common factor of x; second pair, 4
... = x(x -7) +4(x -7) . . . . . . . these terms now have a common factor: (x -7)
... = (x +4)(x -7) . . . . . . . . . . the complete factorization of x² -3x -28