You must FOIL (first, outside, inside, last)
First (The first number/variable of each parentheses must be multiplied together)
(2x - 3y)(4x - y)
Outside (The outside number/variable of each parentheses must be multiplied together)
(2x - 3y)(4x - y)
-2xy
Inside (The inside number/variable of each parentheses must be multiplied together)
(2x - 3y)(4x - y)
-12xy
Last (The last number/variable of each parentheses must be multiplied together)
(2x - 3y)(4x - y)

Now add/subtract each FOIL answer according to its sign
- 2xy - 12xy + 
Combine like terms
+
- 14xy
Hope this helped!
~Just a girl in love with Shawn Mendes