Use the quadratic formula, which is 
The equation is already set up in standard form (which is ax^2 + bx + c), so now you can begin plugging in values.
Comparing the equation with the standard form, you can see that a = 2, b = 3, and c = 20.
After plugging them in, you will begin to notice that the value inside of the square root sign is negative. Whenever this happens, you can take out the negative sign and multiply an "i" variable right before the square root sign. This variable serves as an "imaginary number" and usually occurs whenever there's a negative constant inside of a square root. Nonetheless, you will end up with two answers:
(-3 ± i√71)/4
The plus/minus sign indicates there being two answers.