how do you add when there is a negative exponent?

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a negative exponent basically makes  it a fraction.  So, you can put what you had before in the denominator, then change the exponent to positive, and then in the numerator put a 1.

In a slightly more difficult fashion, 45 + 3^-5 = 45 + 1/(3^5) = 45 + 1/243 = 45 1/243.