First, find a pattern in the relationship between the two numbers in each set:
1*6 = 6, 1 + 5 = 6, 1^n*6 = 6
2*12 = 24, 2 + 22 = 24, 2^2*6, 2^3*3 = 24
3*18 = 54, 3 + 52 = 54, 3^2*6 = 54, 3^3*2 = 54
Sorting through the possible relationships reveals that in each case the first value in (x,y) relates to the second by the equation x^2*6 = y