Two right triangles share a side as follows. What is the distance from C to line AD?
Draw CX from point C perpendicular to line AD, with X on AD.
The length of CX is the distance from C to line AD.
Triangle(CXA) will be a right triangle with CA the hypotenuse.
Since triangle(DAB) is an isosceles right triangle with the right angle at B,
angle(DAB) = 45o.
This makes angle(CAD) = 45o.
For triangle(CAX), sin( angle(CAX) ) = CX / CA.
---> sin(45o) = CX / 10
Finish this equation to find the length of CX, the distance from C to line AD.