Q. Illustrate the Says Law?

With Say's Law, aggregate demand would always be equal to aggregate supply and cross model would be incorrect. 

Keynes's argument as to why Say's Law doesn't apply can be explained in the cross model. According to Say's law, supply creates its own demand. When supply increases, income increases and a higher income creates an equally large increase in demand. Firms and households are stimulated to a higher demand by cuts in real interest rates. Higher aggregate supply will lower the real interest rate and investment and consumption will increase. According to Say's Law, r will fall to the level where total increase in C and I is exactly as large as increase in aggregate supply.

According to Keynes and cross model, this won't happen. When Y increases, C will increase however not as much as Y (and I will not change at all). Aggregate demand won't increase as much as aggregate supply and Say's Law will fail.