Communication of proposed adjustment u/s 143(1)(a) of Income Tax Act, 1961

Hi Sir,

I got the following intimation on my capital gain tax. 

 Communication of proposed adjustment u/s 143(1)(a) of Income Tax Act, 1961.

what should I do now as am not sure about the variance of the amount on capital gain tax, I have calculated from broker website both long and short term capital gain and based on the
 adjustments I have return my tax and paid for the Capital gain from my calculation. 

Now would like to know how they came across the value from Amount as computed just curious on the difference, how to check and validate with that amount. 

If  am not responding within 30 days as mentioned in the letter, they will file and again need to pay additional tax?

Please advise.

Thanks
PPkumar