The speed of an Arabian horse is "4x + 2" miles per hour slower than a thoroughbred horse with a speed of "25x - 5" miles per hour. Find a single expression that represents the speed of an Arabian horse.

The speed of the thoroughbred horse is equal 25x-5 as given in the statement above. Next, since we described the speed of the Arabian horse to be 4x+2 slower than the former's speed, the expression would then become,

speed of Arabian horse = (25x-5) - (4x+2)

= 21x-7