Answer:

To find the value of \( k \) when \( (1, 2) \) is a solution of the equation \( 3x - ky = 5 \), we'll substitute \( x = 1 \) and \( y = 2 \) into the equation and solve for \( k \):

\[ 3(1) - k(2) = 5 \]

Now, let's solve for \( k \):

\[ 3 - 2k = 5 \]

Isolate \( k \) by subtracting 3 from both sides:

\[ -2k = 2 \]

Now, divide both sides by -2 to solve for \( k \):

\[ k = -1 \]

So, when \( (1, 2) \) is a solution of \( 3x - ky = 5 \), the value of \( k \) is \( -1 \).