Answer:
[tex]36[/tex]
Step-by-step explanation:
Given
[tex]x^2 - 12x = 21[/tex]
Required
Make a perfect square
First, get the coefficient (k) of x
[tex]k = -12[/tex]
Divide by 2
[tex]\frac{k}{2} = \frac{-12}{2}[/tex]
[tex]\frac{k}{2} = -6[/tex]
Square this result:
[tex](\frac{k}{2})^2 = (-6)^2[/tex]
[tex](\frac{k}{2})^2 = 36[/tex]
Hence, 36 when added to the left-hand side makes it a perfect square trinomial
