Just line them up like subtracting whole numbers like for example:
Those number that are joined with x can only subtract or be subtracted from numbers that have x also or 0.
Ex.
2x - 1 = 2x - 1
2x -x = x
10x - 6x = 4x
15x² - 10x = 15x² - 10x
15x² - 10x² = 5x²
15x³ - 10x² = 15x³ - 10x²
15x³ - 10x = 15x³ - 10x
15x³ - 10x³ = 5x³
3x - 7 subtracted by 2x + 1
3x - 7
-
2x + 1
= x - 8
3x² + 5 subtracted by x² + 3x + 2
3x² + 0 + 5
-
x² + 3x + 2
= 2x² -3x + 3
