Because it's an arithmetic series, you know you're going to use:
An = A1 + (n-1)d
Where,
An = general term
A1 = first term
n = number of terms
You want the 31st term so you plug everything you know in to the equation
A31 = A1 + (n-1)d
A31 = 26 + (31-1)d
A31 = 26 + 30d
You don't know what the common difference is so you can't finish the problem. Instead, use everything else they gave you to plug in what you know to find what you don't know:
An = A1 + (n-1)d
-226 = 26 + (22-1)d
-226 = 26 + 21d
-252 = 21d
d = -12
Now that you have everything you need, plug it all in to find the 31st term (Also called A31).
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Verified answer
Because it's an arithmetic series, you know you're going to use:
An = A1 + (n-1)d
Where,
An = general term
A1 = first term
n = number of terms
You want the 31st term so you plug everything you know in to the equation
A31 = A1 + (n-1)d
A31 = 26 + (31-1)d
A31 = 26 + 30d
You don't know what the common difference is so you can't finish the problem. Instead, use everything else they gave you to plug in what you know to find what you don't know:
An = A1 + (n-1)d
-226 = 26 + (22-1)d
-226 = 26 + 21d
-252 = 21d
d = -12
Now that you have everything you need, plug it all in to find the 31st term (Also called A31).
An = A1 + (n-1)d
A31 = 26 + (31-1)-12
A31 = 26 + (-360)
A31 = -334
-226 = 26 + 21d
d = -12
31st term = 26+30*(-12) = -334
answer -334