Consecutive terms in this sequence differ by <em>p</em>.
First term: <em>p</em>
Second term: <em>p</em> + <em>p</em> = 2<em>p</em>
Third term: 2<em>p</em> + <em>p</em> = 3<em>p</em>
and so on. It follows that the <em>n-</em>th term satisfies
<em>np</em> = 336
Presumably you meant to say the "sum of the first <em>n</em> terms" is 7224, which is to say
<em>p</em> + 2<em>p</em> + 3<em>p</em> + … + <em>np</em> = 7224
which can be rewritten as
<em>p</em> (1 + 2 + 3 + … + <em>n</em>) = 7224
<em>p</em> (<em>n</em> (<em>n</em> + 1)/2) = 7224
<em>n</em> (<em>n</em> + 1) <em>p</em> = 14,448
Substitute the first equation in the second one and solve for <em>n</em> :
336 (<em>n</em> + 1) = 14,448
<em>n</em> + 1 = 43
<em>n</em> = 42
Now solve for <em>p</em> :
42<em>p</em> = 336
<em>p</em> = 8