Question 1001540

the common difference of the arithmetic sequence 
{{{9}}}, {{{13}}}, {{{17}}}, {{{21}}}, is {{{d=4}}}

as you can see

first term is {{{9}}}
second term is {{{13}}} which is {{{9+4}}}
third term is {{{17}}} which is {{{13+4}}}
fourth term is {{{21}}} which is {{{17+4}}}