Math 7. subtracting linear expressions.

i need with subtracting linear expressions.

(-6x + -6) - (-2x - 9) = ?

Aboat has a speed of 9 mph in calm water. it takes the boat 4 hours to travel upstream but only 2 hours to travel the same distance downstream. which equation can be used to find c, the speed of the current? 2(9 – c) = 4(9 + c) 9 + c = 4(9 – c) 9 – c = 2(9 + c) 4(9 – c) = 2(9 + c)