Question 308839
As always goes for easy school problems, look at the differences between numbers. (you do not have to be a genius to remember this simple rule, always try it first if the answer is not obvious)

differences are

1,1.5,2.25

those are powers of 1.5.

So the next number is {{{6.75+1.5^3}}}
*[invoke explain_simplification "6.75+1.5^3" ]
The next number is {{{10.125+1.5^4}}}
*[invoke explain_simplification "10.125+1.5^4" ]