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Parabolas With One Solution

ACTMAT-4ZX2Y4

Which of the following constants needs to be added to:

$$f(x) = { x }^{ 2 }-6x+4 $$

...in order for the graph to be tangent to the x-axis?

A

$9$

B

$-13$

C

$5$

D

$-9$

E

$32$