Solve the oblique triangle given the information:a=3, b=4, ã=40°

Is ã the angle opposite side a? If so, let's call it A

According to the law of sines,
sin B/b = sin A/a = 0.2143
sin B = 0.8571
B = 60.0 degrees or 120 degrees.

Since you have said it is an oblique triangle, B = 120 and C = 20 degrees

sinC/c = 0.2143, so c = 1.596