So you start by adding the two numbers on either side of your ratio together...
4 + 3 = 7
Then you divide your total number (the book pages) by your total ratio number...
490/7 = 70
Finally, you multiply each side by 70.
4 x 70 = 280 geography
3 x 70 = 210 civics
Plus, you can check your answer by adding the two totals...
280 geography + 210 civics = 490 pages total