placing an arc
"A picture is worth a 1000 words." Post what you have so the solutions you get will make sense.
GrantsPirate
Piping and Mech. Designer
Always save a copy of the drawing before trying anything suggested here.
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If something I wrote can be interpreted two ways, and one of the ways makes you sad or angry, I meant the other one.
In this illustration, the original Circle and Line are in white. Draw a Circle using the TTR option, tangent to the original Circle and Line and at your desired radius [dashed yellow]. Offset the original Circle using the Through option, through the desired endpoint of the Line [dashed blue]. Rotate the [dashed] yellow Circle about the Center of the white Circle, using the Reference option, with the Reference direction being dashed green [to the intersection of Offset dashed-blue Circle and TTR dashed-yellow Circle], and the new direction being the cyan [to the desired endpoint of the Line] -- solid yellow is the resulting position. Trim the solid yellow Circle to an Arc as appropriate.
Kent Cooper, AIA 
Let's assume the radius of the arc you want to create is R. Given the white circle and line create a concentric circle with a radius of the white circle + R yielding the red circle.
Create a circle of radius R at the end of the line (green circle).
Create a circle centered at the intersection of the red and green circles yielding the yellow circle and trim.
Note, there are two solutions represented by the two intersections of the red and green circles.
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BTW, To do a screen capture you can use the Print Screen button on your keyboard to capture the entire screen or Alt-Print Screen to capture the active window. Better yet is to use the Windows 10 Snipping Tool. Just do a search for it.