Rinse shrimp and drain in colander. In large pot, boil water for pasta. Heat butter and olive oil in a skillet over medium heat. Add garlic and reduce heat to low. Sauté garlic until soft. Add shrimp and cook uncovered for approximately 5 minutes on each side, until shrimp turns pink. Remove from heat. In separate saucepan, heat marinara sauce and season to taste with crushed red pepper. Add shrimp to marinara sauce and keep warm. Add pasta to boiling water and cook for three minutes. Drain pasta and place in serving dish. Top with shrimp and marinara sauce.