Hello

The formula is

(a-b)^2 = a^2-2ab+b^2

and it is 4k^2 -36k + 81

compare these to terms

4k^2 = a^2

-2ab = -36k

b^2 = 81

4k^2 = a^2 => a = 2k

81 = b^2 => b=9

Now use the second equality: -2ab = -36k to verify your solution

-2*a*b = -2*2k *9 = -4k*9 = -36k

=> (a+b)^2 = (2k-9)^2