SALEM – A Lynn landscaper who admitted stabbing his former girlfriend and her new boyfriend last year is headed to state prison for up to 10 years.Saul R. Hernandez, 31, who last lived at 3 Albany Terrace, Lynn, changed his plea to guilty last week in Salem Superior Court to charges of mayhem, armed assault with intent to murder and two counts of assault and battery causing serious bodily injury.He was sentenced to serve not less than seven years and not more than 10 years in state prison by Judge Howard J. Whitehead.On the evening of Aug. 31, Hernandez went to Barnard Hawkes Court in Marblehead where his then 28-year-old former girlfriend let him into her home after he asked to use the bathroom. Once inside, Hernandez pretended to look for something, but instead grabbed a knife with a long blade and slashed her in the face and then stabbed her 22-year-old new boyfriend in the chest when he came to her aid.When police arrived at the scene, two police officers at gunpoint had to pry the knife out of his hands and then he tried to escape down the apartment stairway before being taken into custody by force.On two occasions before the incident, Hernandez was ordered by the court to stay away from his former girlfriend, but he had apparently contacted her several times by phone prior to the stabbing incident, authorities said.Four remaining charges against Hernandez in connection with the case for assault and battery with a dangerous weapon, a shod foot, attempted breaking and entering in the nighttime and two counts of resisting arrest were dropped as part of the negotiations.Hernandez, a native of Guatemala, has been held in custody on no bail since being arraigned last November.The judge credited him the 355 days he has spent in jail awaiting trial.