The number was 160 because 5/4•128=160. 5/4 represents inverting 80% and multiplying it by the number to determine how the result became 128. For checking over all you have to do is do 4/5 or 80%•160 which shall give you 128.
The volume of a cylinder is given by the multiplication of the area of the base by the height of the cylinder. π×r²(area of the base) × h (height of the cylinder) ⇔

×

cm²×35cm=12127.5cm³
(r is half of the diameter. Because we are doing the square of r, the cm also are squared.)
Rounding to the nearest tenth is to round the result so that it maintains a number after the point - the result is 12127.5cm³
Your calculator says error because you cant have a square root of a negative number because there is no solution to that maybe recheck your equation and try again
Solve for x in 2nd equation
times -1 both sides
x-5=6y
add 5
x=6y+5
sub
5(6y+5)+4y=-26
30y+25+4y=-26
34y+25=-26
minus 25 both sides
34y=-51
divide both sides by 34
y=-3/2
sub back
x=6y+5
x=6(-3/2)+5
x=-18/2+5
x=-9+5
x=-4
(-4,-3/2) is solution
A negative times a negative is a positive. I am not sure if that's the question, but hopefully that helps if it was.