Answer:
There are two possible values: MN=5 or MN=19
Step-by-step explanation:
The points L and N are collinear, so we can visualize them on a right line as in Figure 1.
First suppose that M is between L and N as in Figure 2. Then we can compute the distance between L and N as LN=LM+MN. Substracting LM from both sides, we obtain that MN=LN-LM=12-7=5.
For the other possibility, suppose that M is not between L and N as in Figure 3. Because LM<LN, it's impossible that M is located further to the right than N. Then M isn't at the right of L. Therefore, M is at the left of L, so L is between M and N, so the distance between M and N is given by MN=ML+LN=LM+LN=7+12=19.
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I hope this helps
