Ricardo's family is on vacation. the first day they drove 348.9 miles. the second day they drove 527.2 miles. the third day they drove 221.7 miles. about how far did ricardo's family drive on their vacation?

This is an exercise in estimation since you ask "about" how far.
348.9 will become 350
527.2 will become 525
and 221.7 will become 220.

350 and 525 are easy to add in your head.  Think:  300 + 500 + 50 + 25 and you can easily get 875.
Now lets break 220 into 200 + 20.  First we'll add the 200 to 875 and get 1075 and finally we will add 20 to that and get 1095.

I do not recommend changing the first number to 300 and the second to 500 and the third to 200.  You will get 1000 but because you rounded too "coarsely" you are almost 100 away from the exact answer.  If you can, do it the way I showed you above.

The linear equation is: y=ax+b