# Factorise the expression: b^2 - 21b + 108

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We'll apply quadratic formula to determine the roots:

b1 = [-(-21)+sqrt((-21)^2 + 4*108)]/2*1

b1 = (21+sqrt9)/2

b1 = (21+3)/2

b1 = 12

b2 = (21-3)/2

b2 = 9

We can write the quadratic expression as a product of linear factors:

b^2 - 21b + 108 = (b - b1)(b - b2)

b^2 - 21b + 108 = (b - 12)(b - 9)