Quadratic Equations Using a Formula

Hi Happy New Year everyone hope you all have had a good one........ I'm doing Maths on New Years Day (Nerd)..... I'm having a bit of trouble with quadratic equations using a formula. I am using the formula to solve the equations but when I substitute the answers back in the equation the answers don't work....

This is the equation :

4²-3x-2=0

I use the formula :

x = -b ± √ b² - 4ac

_____________

2 a

I substitute the letters for the numbers in the formula so that it looks like

x = - - 3 ± √ -3² - 4 x 4 x -2

_____________________

2 x 4

x = 3 + 4.80

________

8

or

x = 3 - 4.80

_________

8

Can someone please tell me where I'm going wrong

I am stating roots correct to 2 dp