the length of the dotted line is measured by find first the length of the upper part of the diagram
now from the triangle on the upper part
9 square + 8 squared = the square of the lower part.
that is X squared = 81. + 64.
x squared = 146
x =. 12. 1 or approximately 12
therefore the dotted line is
y squared = 10 ². +. 12²
y squared = 100+144
y squared = 244
y = the square root of 244
y is equal to 15.6