To determine the value of \( h \) in the function \( f(x) = x + h \) that describes the translation from the point \( (5, 6) \) to the point \( (3, 6) \), we need to focus on the x-coordinate.
The point is translated from \( x = 5 \) to \( x = 3 \). We can express this translation in terms of the function \( f(x) \):
\[ f(5) = 3 \]
Substituting \( x = 5 \) into the function, we have:
\[ f(5) = 5 + h \]
Setting this equal to the new x-coordinate after the translation:
\[ 5 + h = 3 \]
Now, we solve for \( h \):
\[ h = 3 - 5 = -2 \]
Thus, the value of \( h \) is
\[ \boxed{-2} \]