That's easy, you just multiply.
For example, when its 3 * 1 = 3, but here there are letters/variables that hold number which you don't know and that's the reason you write letter with it showing you are multiply it instead of writing 3 * a, simple for would be 3a.
a) 3a
b) 3a
c) 4x
d) 3pq
e) 7fe
f) zwy