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