Answer:
15
Step-by-step explanation:
If the bigger triangle has the two congruent sides measuring 20, then the ratio of the bigger to the smaller is 20/5.6 This means that the ratio between the third side of the bigger to the third side of the smaller is 20/5.6
Setting a ratio:
20/5.6=x/4.2
Cross multiplying: 84=5.6x
x = 84/5.6 -->12/0.8 --> 15
x = 15