Answer :

To find the value of [tex]\( k \)[/tex] given that one solution of the quadratic equation [tex]\( x^2 - kx + 21 = 0 \)[/tex] is [tex]\( x = 3 \)[/tex], we can follow these steps:

1. Substitute the known solution into the quadratic equation:

Given that [tex]\( x = 3 \)[/tex] is a solution, substitute [tex]\( x = 3 \)[/tex] into the equation:
[tex]\[ (3)^2 - k(3) + 21 = 0 \][/tex]

2. Simplify the equation:

Calculate [tex]\( 3^2 \)[/tex]:
[tex]\[ 9 - 3k + 21 = 0 \][/tex]

3. Combine like terms:

Combine the constant terms on the left-hand side:
[tex]\[ 30 - 3k = 0 \][/tex]

4. Solve for [tex]\( k \)[/tex]:

Isolate [tex]\( k \)[/tex] on one side of the equation. To do this, first move [tex]\( -3k \)[/tex] to the right-hand side:
[tex]\[ 30 = 3k \][/tex]

Next, divide both sides by 3 to solve for [tex]\( k \)[/tex]:
[tex]\[ k = \frac{30}{3} \][/tex]

Therefore,
[tex]\[ k = 10 \][/tex]

So, the value of [tex]\( k \)[/tex] is [tex]\( 10 \)[/tex].