How to find the common difference and term 1 in an arithmetic sequence

I need to know how to find the common difference and term 1 the arithmetic sequence. Term 3 is 16 and Term 7 is40

t3 = a +2d = 16 (1)
t7 = a + 6d = 40 (2)
(2) - (1) gives 4d = 24 so d = 6
substitute d = 6 into (1)
a + 2(6) = 16
a = 16 - 12
= 4
So t1 = 4 & the common difference is 6

tռ = t₁ + (n-1)k for n = 2, 3, 4, ...

t₁ + 6k = 40
-(t₁ + 2k = 16)
-------------------
4k = 24

k = 6

t₁ + 2(6) = 16
t₁ = 4