We know that the product of the G.C.D. and L.C.M. of two numbers is equal to the product of the numbers themselves. Let the two numbers be \( a \) and \( b \), and we are given:
\[
\text{G.C.D.}(a, b) = 12, \quad \text{L.C.M.}(a, b) = 252, \quad a = 36.
\]
Then, we can use the relation:
\[
\text{G.C.D.}(a, b) \times \text{L.C.M.}(a, b) = a \times b.
\]
Substitute the known values:
\[
12 \times 252 = 36 \times b \quad \Rightarrow \quad 3024 = 36 \times b \quad \Rightarrow \quad b = \frac{3024}{36} = 84.
\]