Quoting i means to divide

so the nth term is really the 10th term, and we know that's 57, thus

so, that's the common difference... .so you'd surely know what the 3rd term is, notice the first one is 102.
Use PEMDAS:
P Parentheses first
E Exponents (ie Powers and Square Roots, etc.)
MD Multiplication and Division (left-to-right)
AS Addition and Subtraction (left-to-right)
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You don't have the graph icon here, so we'll have to graph this parabola without it.
Your parabola is y = -x^2 + 3., which resembles y = a(x-h)^2 + k. We can tell immediately that this parabola opens down and that the vertex is (0,3).
Plot (0,3). Besides being the vertex, this point is also the max. of the function.
Now calculate four more points. Choose four arbitrary x-values, such as {-2, 1, 4, 5} and find the y value for each one. Plot the resulting four points. Draw a smooth curve thru them, remembering (again) that the vertex is at (0,3) and that the parabola opens down.
1 + 2^1 = 3
3 + 2^2 = 7
7 + 2^3 = 15
15 + 2^4 = 31
31 + 2^5 = 63
63 + 2^6 = 127
so the next two terms in the sequence 1, 3, 7, 15, 31, is 63 and 127.