When \( y \) varies directly with \( x \), it can be expressed with the equation:
\[ y = kx \]
where \( k \) is the constant of proportionality. You are provided with \( y = 35 \) when \( x = 5 \). We can substitute these values into the equation to find \( k \).
\[ 35 = k \cdot 5 \]
To solve for \( k \), divide both sides by 5:
\[ k = \frac{35}{5} \]
Calculating this gives:
\[ k = 7 \]
Thus, the constant of proportionality \( k \) is \( \boxed{7} \).