Answer:
The answer to this question is
The lattice energy of the salt AB is ≅ -2600 kJ/mol rounded to three significant figures
Explanation:
To solve this we list out the known variables and the required unkown, then we look for the appropriate relation between the known and the unknown thus
Lattice energy of KI = -649 kJ/mol
Latice energy of salt AB = unknown
Charge on A²⁺ =+2
Charge on B²⁻ = -2
radius of A²⁺ = radius of K⁺
radius of B²⁻ = radius of I⁻
the Born-Lande equation for lattice energies is as follows

Where to the above question, the following terms are important
z+ = positive ion charge
z− = negative ion charge
r₀ → distance between the positive and the negative ions;
Taking all variables of KI and AB as identical we have.

Where radius of A²⁺ = radius of K⁺ and
radius of B²⁻ = radius of I⁻ = r₀
we have

As for AB z⁺ = +2 and z⁻ = -2 we have

= 4 ×(-649 kJ/mol) = -2596 kJ/mol
≅ -2600 kJ/mol rounded to three significant figures