a) When the magnification is positive, then the image is virtual and erect. In this case, the magnification has a positive sign, so the nature of image is virtual and erect.
b) The value of magnification gives here is 2.5 (Which is more than 1). So, the image is larger than the object or magnified. A virtual, erect and magnified image can be formed only by a convex lens, therefore, the nature of lens is convex.
