Would someone explain to me where to start.

Which is an equation for the nth terms of the sequence 12,15,18,21

White shapes and black shapes are used in a game. some of the shapes are circles. while the rest are squares. the ratio of the number of white shapes to the number of black shapes is 5: 11. the ratio of the number of white cicrles to the number of white squares is 3: 7. the ratio of the number of black circles to the number of black squres is 3: 8. work out what fraction of all the shapes are circles.

Use the drop-down menus to complete the statements to match the information shown by the graph.