To strengthen your left bicep, try doing single dumbbell curls on an incline bench. This isolates your left bicep and forearm, so you're not using your back to lift the weights.
For your left tricep, lie flat on your back with a dumbbell in your left hand. Extend your arm straight in the air, perpendicular to your body. While using your right hand to stabilize your upper left arm (kinda hold onto your bicep), lower the weight and bend your elbow as low as you can hold up, and then raise your arm up again. Sorry I can't remember the name for this exercise.