I need help please and thank you

  3.33×10⁴

Step-by-step explanation:

You can use the hint, or you can use a common exponent. Here is the latter case.

  (3.9×10⁴) -(5.7×10³) = (3.9×10⁴) -(0.57×10⁴) = (3.9 -0.57)×10⁴

  = 3.33×10⁴

With a little practice, you can see the effect on exponents of moving the decimal point. Here, we have ...

  5.7×10³ = (0.57×10)×10³ = 0.57×(10×10³) = 0.57×10⁴

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When adding or subtracting in scientific notation, all that is required is that the exponents of all the numbers be the same. The hint says make all the exponents be 0. Above, we have chosen to make them all be 4. You could also use 3:

  (3.9×10⁴) -(5.7×10³) = (39×10³) -(5.7×10³) = (39 -5.7)×10³

  = 33.3×10³ = 3.33×10⁴

it's equivalent, so we can say that its equal.

let's call the denominator x and the numerador is 18 less then denominator, x - 18.

multiply

2.x = 5.(x - 18)

2x = 5x - 90

90 = 5x - 2x

90 = 3x

x = 90/3

x = 30

so: