where is the centre of the yellow region?

I cant find the centre. Please give me a suggesion how can i draw the circle or which tool can i use to draw it.

4 Answers

Somewhere here... (Check the red dot)

For drawing the circle, start a new sketch on the desired face/plane/etc.
Then draw the circle using the circle sketch tool.

Here is the file

SolidTweaks is right. You don't need to know or care where the center of that fillet is, it's simply a 25mm "face" fillet across the circular features of the model.

You do not need a centre to fully define the skech. When you sketch properly, you will have to constraint the Cirlce (R25) tangential to the other 2 circles (R20). If you have defined all the other dimesions correctly you do not need to find the centre, in this case.