Maths Problem help :)

Hi, I'm stuck on a maths question...
Find the sum of the 20 terms of the arithmetic series with first term 3 and last term 17. Find also the common difference of the series?
thanks :)

General formula for S_n:
S_n = (n/2)(a_1 + a_n)

S_20 = (20/2)(3 + 17) = 200

General formula for a_n:
a_n = a_1 + (n-1)d

a_1 = 3
a_20 = 17

Applying the formula,

17 = 3 + 19d
14 = 19d
14/19 = d