Let's put more details in the figure to better understand the problem:
To be able to determine DC, let's treat this as two similar triangles and apply ratio and proportion.
We get,
[tex]\text{ }\frac{\text{ AB}}{\text{ OB}}\text{ = }\frac{\text{ EF}}{\text{ OE}}[/tex][tex]\frac{18}{12}\text{ = }\frac{x}{\text{ 10}}[/tex][tex]\frac{18\text{ x 10}}{12}\text{ = }x[/tex][tex]\frac{180}{12}\text{ = }x[/tex][tex]15\text{ = }x[/tex][tex]\text{ FE = 15}[/tex]
If a diameter or radius is perpendicular to a chord, then it bisects the chord and its arc. Therefore, we can say that FE = ED.
Determining the length of FD, we get:
[tex]\text{ FE + ED = FD}[/tex][tex]\text{ FE + FE = FD}[/tex][tex]\text{ 15 + 15 = FD}[/tex][tex]\text{ FD = 30}[/tex]
Therefore, FD = 30
