Based on my learnings, subtraction of algebraic expression is basically subtraction of integers [change the sign of the subtrahend and proceed to addition] but the variables will stay at ease.
3x - 2x = x [no need to write 1x because it is understood 1x = x]
(2x^2 + 5x + 6) - (x^2 - 3 - 4) =
= (2x^2 + 5x + 6) + (-x^2 + 3 + 4)
= (x^2 + 8x + 10)
See how the variables remain at ease plus the rule of subtraction.