Answer:
EF=40
Step-by-step explanation:
DC=9, AB=x+11, EF=?
EF=x-20
EF=(AB+DC)/2
x-20=(x+11+9)/2
x-20=(x+20)/2
x-20=x/2+10
x-x/2=10+20
2x/2-x/2=30
x/2=30
x=30*2
x=60
EF=x-20=60-20=40
