Sadie wrote the expression correctly. She used the associative property correctly.
Answer:
-10
Step-by-step explanation:
1. First, let's simplify both sides of the equation.
2. To find the missing value to make the equation have infinitely many solutions, we need to make both sides of the equation equal.
3. Given that information, means that 4x - 10 has to equal 4x + (-10) because:
4. Now, with the information in #2, let's see if it's true:
-

-

- (0 = 0 means that the equation has infinitely many solutions)
Therefore, the missing number is -10.
Answer: 
<u>Add 3x to both sides</u>
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<u></u>
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<u>Subtract 15 from both sides</u>
<u></u>
<u></u>
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<u>Divide both sides by 5</u>
<u></u>
<u></u>