If a concave and convex lens of equal focus are

When two lenses are kept in contact, powers of the lenses are simply added.
Power of a lens is positive for convex lens and negative for concave lens.
Numerically power of a lens is given by $\frac{100}{f}$ were f is focal length in centimeters.
As numerically, focal length is same but opposite in size, combined power will be
Zero i.e. focal length is $\mathrm{\infty }$ i.e. it is just equivalent to plane glass,