To find the answer, we need to first understand the feature of similar triangle, where the sides of two triangle would be of the same ratio.
In this case, the equation that can be set to show that they have the same ratio:
MN/ON = LM/OP
LM and OP are used to find the ratio as they are given.
MN/12 = 42/14
MN = (42/14) x 12
MN = 36cm
Therefore, the answe is 36cm.
Hope it helps!