One thing for sure it is not b,d,c its a
The given equation is: 
To find the line perpendicular to it, we interchange coefficients and switch the signs of one coefficient.
The equation to a line perpendicular to it is:
$ 2y-x=c$
where, $c$ is some constant we have determine using the condition given.
It passes through $(2,-1)$
Put the point in our equation:
$2(-1)-(2)=c$
$c=-2-2$
$c=-4$
The final equation is:
$\boxed{ 2y-x=-4}$
Let V, be the rate in still water and let C = rate river current
If the boat is going :
upstream, its rate is V-C and if going
downstream, its rate is V+C,
But V = 5C, then
Upstream Rate: 5C - C = 4 C
Downstream rate: 5C+C = 6C
Time = distance/Rate, (or time = distance/speed) , then:
Upstream time 12/4C and
Downstream time: 12/.6C
Upstream time +downstream time:= 2h30 ' then:
12/4C + 12/.6C = 2.5 hours
3/C + 2/C = 5/2 (2.5 h = 5/2)
Reduce to same denominator :
5C = 10 and Rate of the current = 2 mi/h
X^2 + y^2 = 8
X-y=0 so x = y
replace x = y into X^2 + y^2 = 8
y^2 + y^2 = 8
2y^2 = 8
y^2 = 8/2
y^2 = 4
y = - 2 and y = 2
because x = y
so x = - 2 and x = 2
solutions:
x= - 2 and x = + 2
y= - 2 and y = + 2
I believe that a flat surface continuing in all directions is called a plane.
