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I started going to the gym about a month ago. A gym instructor drew out a program for me and my program and the current wieghts I am using are:

warm up and cool down:
2 sets 24 reps Sit Ups
2 sets 24 reps Back Extensions

routine:
60 lbs 3 sets 10 reps Leg Extension
45 lbs 3 sets 10 reps Leg Curl
70 lbs 3 sets 10 reps Leg Press
60 lbs 3 sets 10 reps Front Pulldown
40 lbs 3 sets 10 reps Rotary Chest
20 lbs 3 sets 10 reps Seated Shoulder Press
50 lbs 3 sets 10 reps Tricep Pushdown
30 lbs 3 sets 10 reps Barbell Curl

Are there any muscle/s groups I am missing out?
Are the weights I'm using balanced or am I training a certain part of my body more than the rest?
With this program, where can I go on next? What other exercises can I incorporate in this routine?

I am an ectomorph. My current body weight is at 120lbs and I am 5ft.9inches tall. I am underweight and I am aiming to gain weight. I go to the gym 3x a week, usually spend about 1.5hrs to do all of those listed above. I do want a big body but not as big as those who compete..