Answer:
The speed from the observer is 8 mph.
Step-by-step explanation:
When the boat is going downstream with full force, its speed will be the one the oarsman can row on still water added by the flow of the river. In this case he can row the boat at speed of 3 mph, while the stream is 5 mph, so the speed from the observer standpoint is:

The speed from the observer is 8 mph.
 
        
             
        
        
        
Step-by-step explanation:
take the right triangle
15^2 = 9^2 + x^2
225 = 81 + x^2
x = 12
area of square b = 144
 
        
             
        
        
        
Answer:
9 cm
Step-by-step explanation:
hope it helps<3
 
        
             
        
        
        
(b⁷/4⁵)⁻³
= (4⁵/b⁷)³
= 4¹⁵/b²¹
Therefore answer is option b
Must click thanks and mark brainliest