"d-orbitals with opposite values of m[l] combined giving real standing waves with boundary surfaces"?

I've been asked

"Show that d orbitals with opposite values of m[l] may be combined in pairs to give real standing waves with boundary surfaces as shown in Fig. 4.15 and with forms that are given in eqn 4.19."

Here are the relevant information:

http://i.imgur.com/IcC9yN0.jpg

I've also been handed a hint-sheet:

"There are five d-orbitals (l=2) for n>=3. All except the orbital with m[l]=0 are complex; however it is more common to display them as their real components, i.e. the following linear combinations:

d[z^2]=d[0]

d[x^2-y^2]=1/sqrt(2) (d[+2]+d[-2])

d[xy]=1/(i*sqrt(2)) (d[+2]-d[-2])

d[yz]=1/(i*sqrt(2)) (d[+1]+d[-1])

d[zx]=1/sqrt(2) (d[+1]-d[-1])

The value of m[l] is given as the subscript in the above. The purpose of this exercise is to check, that the above normalized sums of d-orbital functions with m[l] of opposite signs are real (Eqn. 2-5)"

(Sorry for formatting. Take [] to mean sub-script)

Got to be honest, I'm lost... I've got no clue and don't know where to even start.