Explanation:
Because the two triangles are similar the order of the angles are the similar angles as well so
Q is similar, or in this case because they are equal, to M
And R is equal to N
So line PQ is similar to PM.
You would then find the rate that triangle PMN is smaller then PQR
So it would be 70 / 10 = 7
And line RP and NP are similar and because we have the length of of the smaller triangle, you would just multiply 7 by 7
And 7 times 7 is 49
So the missing length is 49.