Sabrina has 21 tiles overall.
Green tiles: x
Red tiles: 2x
Blue tiles: 2 * 2x = 4x
We can write it all as:
21 = x + 2x + 4x
21 = 7x / ÷ 7 (both sides)
x = 3
Doublecheck:
Green tiles: 3
Red tiles: 2 * 3 = 6
Blue tiles: 2 * 6 = 12
All tiles: 3 + 6 + 12 = 21
Correct :)