8 x² + 14 x + 5 = 0
2 ( 4 x² + 7 x ) = - 5
2 ( 4 x² + 7 x + 49/16 - 49/16 ) = -5
2 ( 4 x² + 7 x + 49/16 ) - 98/16 = - 5
2 ( 2 x + 7/4 )² = - 5 + 98/16
2 ( 2 x + 7/4 )² = -80/16 + 98/16
2 ( 2 x + 7/4 )² = 18/16 = 9/8 / : 2
( 2 x + 7/4 )² = 9/16 / √
2 x + 7/4 = 3/4
2 x = 3/4 - 7/4 = -1
x = - 1
and 2 x + 7/4 = -3/4
2 x = -7/4 - 3/4 = - 10/4= -5/2
x = - 5 / 4