The answer is: " 12 months " .
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Explanation:
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(129 − 45) / 7 = 84 / 7 = 12 mos.
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Step 1. READ THE QUESTION CAREFULLY!!!!!!
Step 2. Turn the problem into equations.
Step 3. Substitute/Eliminate variables.
Step 4. Find the first variable.
Step 5. Replace the variable1 with its correct number.
Step 6. Get your variable2.
Step 7.Do steps 3-4 until you have all the variables.
Step 8. Double check.
And you are done!
the constant of variation or namely its slope will be

first pemdas
parentheses exponents multiply divide addition and subtraction
so the answer is 7