How to subtract rational expressions.

According to modern science, earth is about 4.5 billion years old and written human history extends back about 10,000 years. suppose the entire history of earth is represented with a 10-meter-long timeline, with the birth of earth on one end and today at the other end.

Twice the sum of a number and 4 is equal to three times the difference of the number and 7. find the number.

Can you me find the missing length. i attached an image.