Sequencing question help

For an arithmetic sequence, let a1 = 5 and d = -2
Find: an, a10, s10

For an arithmetic sequence, let a2 = 15 and a10 = 10
Find: d, a1

Since a1=5, d= -2

an = a + (n-1)d
Thus,
an = 5 +(n-1) (-2)
-> an = 5 -2n + 2
Thus, an =7 - 2n

Similarly, find out a10 by substituting n=10.

Now, Sn = n/2 (2a + (n-1) d ) ... So, just substitute the values, and you get :

S10 = 10/2 ( 2*5 + (10-1) * (-2))
=> S10 = 5 ( 10 -18)
=> S10 = 5 * (-8) = - 40

Similarly, apply these formula's for the second problem also :)