Help! Please explain also. Thank you

Answer:
17A) 52mm
17B) 132mm
17C) 67mm
18.) 34
Explanation:
For 17)
Each distance is determined by the total distances surrounding it. For example, 17A is 73mm - 21mm, which is 52mm.
For 18)
Set up the problem as an algebraic expression:
a + b + c + d = 115
Since you know 3 of the values, you can plug them in and subtract them from 115, which turns into 34.