Each letter in these equations stands for a movie.
a = b + c
d = e - c
e = f × b
Identify the movies 'a' to 'f' using these directors.
The pictured directors are (in order of presentation):
Alfred Hitchcock, John Ford, George Gallo, Ron Howard, Joel Schumacher, and Dan Trachtenberg.
However, they do not equate to the letters a-f in this order. To work this out, we need to look at...
...movies by these directors which have numbers in their titles. Specifically, we need the following:
a: The Number 23 (Joel Schumacher)
b: Apollo 13 (Ron Howard)
c: 10 Cloverfield Lane (Dan Trachtenberg)
d: 29th Street (George Gallo)
e: The 39 Steps (Alfred Hitchcock)
f: Three Mounted Men, or, alternatively, Three Jumps Ahead (John Ford)
This then satisfies all the equations...
...when the numbers from the titles are used in place of the variables:
a = b + c
d = e - c
e = f × b
23 = 13 + 10
29 = 39 - 10
39 = 3 x 13