Question 875422
boat   	speed	x   	km/h		Time forward + time return = hours												
current     	speed	3	km/h														
																	
against	current     	x-	3	km/h													
with	current     	x+	3	km/h													
																	
Distance=	4	miles															
																	
Time against + time with  =			5/3	hours													
t=d/r																	
																	
4	/(	x   	+	3	)	+	4	/(x	-	3	)	=	5/3				
																	
LCD =	(x	-	3	)	(	x   	+	3	)								
4	*(	x   	-	3	)	+	4	(x	+	3	)	=	5/3	(x^2	-	9	)
4	x   	-	-12	+	4	x   	+	12		=	5/3	(	x ^2		-	9	)
8	x   	=	5/3	x ^2		-15											
5/3	x ^2		-8	x   	-	15	=	0

5x^2-24x-45=0

Find the roots of the equation by quadratic formula							
							
a=	5	,	b=	-24	,	c=	-45
							
b^2-4ac=	576	+	900				
b^2-4ac=	1476						
{{{	sqrt(	1476	)=	38.42	}}}		
{{{x=(-b+-sqrt(b^2-4ac))/(2a)}}}							
{{{x1=(-b+sqrt(b^2-4ac))/(2a)}}}							
x1=(	24	+	38.42	)/	10		
x1=	6.24						
{{{x2=(-b-sqrt(b^2-4ac))/(2a)}}}							
x2=(	24	-38.42	) /	10			
x2=	-1.44						
Ignore negative value							
boat   	speed	6.242	km/h