To add and subtract monomials, binomials, and trinomials, just remember these simple steps:

1. Like Terms: You can only combine terms that have the same variable and exponent.

   For example:

   • When you add $3x$ and $5x$, you get $8x$.
   • When you subtract $3y^2$ from $2y^2$, you get $-y^2$.

2. Grouping: If you're working with bigger expressions, it helps to line up similar terms.

   For example, with $3x^2 + 4x + 2x^2 - 5x$, you can group the like terms together:

   • First, combine $3x^2$ and $2x^2$ to get $5x^2$.
   • Then, combine $4x$ and $-5x$ to get $-x$.

So, it would look like this:
$(3x^2 + 2x^2) + (4x - 5x) = 5x^2 - x$.

Once you practice a bit, it becomes pretty simple!