Do you need to reproduce this drawing?
First, draw AB with a ruler.
Take your compass, draw an arc with length AB with centre A.
Do the same with centre B.
Where the arcs cut, it is the vertex C.
Then, draw AC and BC with a ruler.
Bisect any of the two sides of the triangle and join the lines of bisection.
Where they meet indicate the centre of the triangle and at the same time the centre of the circle.
Put your compass on the centre of the circle to point D, which is halfway between A and B, (or measure 4.8 units) and draw a circle.
There you are.
I hope that it's what you were looking for.