sun of n terms of AP = N/2{2a+(n-1)*d} where a= first term .d=common difference ,n= number of terms

now put values in the formula a=6, d=6 , n=?,sum=1800

N/2{2*6+(n-1)*6}=1800

$n^{2}+n-600=0$

solving the quadratic equation you will get 24 terms so answer is 24