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

a) [[$ a = 2 $]], [[$ b = 3 $]] and [[$ c = 1 $]]

[[$ x = $]]

b) [[$ a = 1 $]], [[$ b = 2 $]] and [[$ c = –8 $]]

[[$ x = $]]