Confuse what am I doing?

(25-a) (2a+3)= 40     Multiply it out

50a + 75 - 2a^2-3a = 40     and collect 'like' terms

-2a^2 +47a +35 = 0    use quadratic formula\(x = {-b \pm \sqrt{b^2-4ac} \over 2a}\)

 

-47+-sqrt(2209+280)    /   -4     =     - 47+-49.8898    /-4   =  24.222   or  -.7225