Isolate the variable by dividing each side by factors that don't contain the variable.
y = 2 − x
3x − y = 9
y = −2x + 11
3x − (−2x + 11) = 9
3x + 2x - 11 = 9
5x = 9 + 11
5x = 20
x = 4
y = −2x + 11 = -2*4+11 = 3
<span>(4, 3)</span>
The number is 5. All you have to do is make an equation based on the information you have, x will represent the missing number so, x to the power of 10 divided by x to the power of 7 is equal to 125, then solve.