Two unknowns in Arithmetic Progression I have a problem in my maths book which says 

Find the arithmetic sequence in which $T_8 = 11$ and $T_{10}$ is the additive inverse of $T_{17}$

I don't have a first term of common difference to solve it, so I managed to make two equations to find the first term and common difference from them. Here they are.
first equation $a + 7d = 11$
second equation since $T_{10} + T_{17} = 0$
therefore $a+9d + a+16d = 0 ~~\Rightarrow~~ 2a + 25d = 0$
So what I did is subtracting the first equation from the second one to form this equation with two unknowns $a – 18d = 11$
This is what I came up with and I can't solve the equations, any help?

It's simple .
a+7d=11
And second eq is 2a+ 25d.
Multiply the first eq by cofficient of a from second eq . i.e.,
2a+14d=22 and
 2a+25d=0 
By subtracting both equations we get 
d=-2.
