Answer :

ooh, a puzzle so we notice that the pairs are
(1,3)
(2,12)
(3,27)
(4,48)
(5,75)
we notice also that
1 times 3=3
2 times 6=12
3 times 9=27
4 times 12=48
5 times 15=75

the numbers go up by muliplues of 3
3,6,9,12,15
3=1 times 3
6=2 times 3
9=3 times 3
12=4 times 3
15=5 times 3
1st one is 1 times 1 times 3
2nd one is 2 times 2 times 3
3rd one is 3 times 3 times 3
4th one is 4 times 4 times 3
5th one is 5 times 5 times 3
so the equation by trial and error is
if the number is x then the equation is
x times (x times 3) or using associative propery which is a(bc)=(ab)c, the equaiton is

3x^2 which means 3 times (x^2)

so the equation is y=3x^2