Have you ever learned to solve equations? If there is, you can understand it this way. . .

Suppose Yingying's age was X three years ago, then Grandpa's age was 7x.

Three years later, Yingying's age this year is x+3, and Grandpa's age is 7x+3.

because

The sum of the ages of Yingying and Grandpa this year is 70.

So it can be expressed as x+3+7x+3=70.

8x+6=70

8x=64

x=8

Because Yingying's age was x=8 three years ago, Yingying is 1 1 year old this year, and Grandpa was 7x=56 three years ago, so Grandpa is 59 years old this year.