Answer: x = 16.8
==============================================
Work Shown:
x/21 = 12/15 ... see note below
15x = 21*12 ... cross multiply
15x = 252
x = 252/15 .... divide both sides by 15
x = 16.8
Note: x is opposite the double arc marking and 21 is opposite the single arc marking. Similarly, 12 is opposite the double arc and 15 is opposite the single arc. This order is important to set up the proportion.
