To solve this equation, we first need to simplify both sides of the equation.

Starting with the left side:

x^2 + 3x - (1 - 2x^2)
x^2 + 3x - 1 + 2x^2
3x^2 + 3x - 1

Now simplifying the right side:

3x^2 - x + 4

Now, we have the simplified equation as:

3x^2 + 3x - 1 = 3x^2 - x + 4

Next, we need to isolate the variable terms on one side of the equation:

3x^2 - 3x^2 + 3x + x = 4 + 1
4x = 5

Divide both sides by 4:

x = 5/4

Therefore, the solution to the equation is x = 5/4.