Remember that
If two triangles are similar, then the ratio of its corresponding sides is proportional and its corresponding angles are congruent
so
Verify if triangle ABC is similar to triangle DEF
step 1
Find out the measure of angle C in triangle ABC
we have that
A+B+C=180
85+56+C=180
C=39 degrees
that means
Corresponding angles of triangle ABC and DEF are congruent
triangles are similar
step 2
If triangles are similar
then
AB/DE=BC/EF=AC/DF
so
BC/EF=AC/DF
substitute given values
24/6=20/DF
solve for DF
4=20/DF
DF=20/4
DF=5 units