Answer: 20
Explanation:
With 2 secants, it's the inside of the secant times the whole secant for one line = same on other side but for other secant
EF(EF+GF)=ED(ED+CD) EF=18; GF=3+CD; ED=19; CD=CD
18(18+3+CD)=19(19+CD) substitute and simplify and distribute
18(21+CD)=361+19CD
378+18CD=361+19CD
CD=17
GF=3+CD substitute
=3+17
=20