1: x = x: 81 what number can replace x?

Multiply 1/x = x/81 by 81x and you get

... 81 = x²

Taking the square root gives

... x = ±9

Either 9 or -9 can replace X to make the proportion statement true.

The equations that are used to determine the values of x and y are 3x - y = 9 and 4x + y = 5. According to the degrees of freedom rule, the number of unknown variables should be equal to the number of independent equations in order to solve the problem. In this respect, we can use elimination in order to solve the equations

3x - y = 9
4x + y = 5

we add these two in order to eliminate y, thus,

7x = 14
x = 2 

from equation 1, 3*2 - 9 = y ; y = -3. hence the solution set is equal to (2,-3). The answer to this problem would have to be C.