SOLUTION: I am confused about grouping. x2-11x+28=0 I know (x- )(x+ ) but...which number am I trying to break down? They are not divisible.

I am confused about grouping.
x² - 11x + 28 = 0 
I know (x- )(x+ )
but...which number am I trying to break down? They are not divisible. 

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There are four cases to learn when the coefficient of x² is 1:

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I. When you want to factor

x² + Ax + B

where A and B are positive integers

1. Think of two positive integers which have product B and sum A
2. If those positive integers are C and D, then factorization is
   (x + C)(x + D)

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II. When you want to factor

x² - Ax + B

where A and B are positive integers

1. Think of two positive integers which have product B and sum A
2. If those positive integers are C and D, then factorization is
   (x - C)(x - D)

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III. When you want to factor

x² + Ax - B

where A and B are positive integers

1. Think of two positive integers which have product B and difference A
2. If those positive integers are C and D, where C is the larger, then
   factorization is
   (x + C)(x - D)

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IV. When you want to factor

x² - Ax - B

where A and B are positive integers

1. Think of two positive integers which have product B and sum A
2. If those positive integers are C and D, where C is the larger,
   then factorization is
   (x - C)(x + D)

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Yours is case II.

x² - 11x + 28

where 11 and 28 are positive integers

1. Think of two positive integers which have product 28 and sum 11
2. Those positive integers are 7 and 4, thus the factorization is
   (x - 7)(x - 4)

Edwin