Calculate the value of the expression [[$ x=\dfrac {-b\pm \sqrt {b^{2}-4ac}} {2a} $]] , when

a) [[$ a = 5 $]], [[$ b = 3 $]] and [[$ c = -8 $]]

Answer: [[$ x $]] =

b) [[$ a = 2 $]], [[$ b = -4 $]] and [[$ c = 2 $]]

Answer: [[$ x $]] =