Answer:
18km/h
Step-by-step explanation:
if the riverboat is already going 15 km/h and the current is 3km/h then you should add and you get 18 km/h
x=6
5x+9-3x=21
5x-3x=21-9
2x=12
a = 11
distribute and combine like terms
6a + 18 = 4a + 40
2a = 22
a =11