To find the constant of proportionality \( k \) in the variation where \( y \) varies directly with \( x \), we can use the formula:
\[ y = kx \]
We know that \( y = 35 \) when \( x = 5 \). Plugging these values into the equation, we get:
\[ 35 = k \cdot 5 \]
To solve for \( k \), divide both sides by 5:
\[ k = \frac{35}{5} = 7 \]
Therefore, the constant of proportionality \( k \) is \( 7 \).