4x + y = 6 -5x - y = 21 solve for both x and y using substitution.

Solve the following system using substitution:
{y - 4 x = 6
{-5 x - y = 21

In the first equation, look to solve for y:
{y - 4 x = 6
{-5 x - y = 21

Add 4 x to both sides:
{y = 4 x + 6
{-5 x - y = 21

Substitute y = 4 x + 6 into the second equation:
{y = 4 x + 6
{-9 x - 6 = 21

In the second equation, look to solve for x:
{y = 4 x + 6
{-9 x - 6 = 21

Add 6 to both sides:
{y = 4 x + 6
{-9 x = 27

Divide both sides by -9:
{y = 4 x + 6
{x = -3

Substitute x = -3 into the first equation:
{y = -6
{x = -3

Collect results in alphabetical order:
{x = -3, y = -6

Answer

1 number

explanation

if we use a graphing calculator, there is only one such number that satisfies the conclusion of the mean value theorem, that is, a number such that

 

where the right-hand side is the average rate of change of f on the interval [0.5,0.75], which is the slope of the secant line.

in the attached graph, the black line is the secant line which has its slope as the average rate of change of f on [0.5,0.75], while the orange line is the only tangent line to the graph of f in the interval that satisfies the mvt conclusion.

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