When 21 is subtracted from the square of a number, the result is 4 times the number. Find the positive solution.

Let x = your number.

x^2 - 21 = 4x. Subtract 4x from both sides.

x^2 - 4x - 21 = 0. Factor.

(x - 7) (x + 3) = 0. Set each term to 0.

x + 3 = 0. Subtract 3 from both sides.

x = -3. This is the negative solution.

Set the other term equal to zero.

x - 7 = 0. Add 7 to each side.

x = 7. This is the positive solution.

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guess at question:

a vertical line intersects the graph of g(x,y) = 0 at two points. what conclusion follows about the function f(x) implicitly defined by g(x,f(x)) = 0?

the domain of f(x) cannot include the x coordinate of any such vertical line.

informally, f(x) is not a function, and

g(x,y)=0 cannot be solved for y uniquely.

the relation r = {(x,y) such that g(x,y) = 0} is a function if

for all (a,b) ∊ r and (c,d) ∊ r, a=c implies b=d.

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