Given,
The nth term of the A.P = (-4n + 15)
So, by putting n = 1 and n = 20 we can find the first ans 20th term of the A.P
a = (-4(1) + 15) = 11
And,
a20 = (-4(20) + 15) = -65
Now, for find the sum of 20 terms of this A.P we have the first and last term.
So, using the formula
Sn = \(\frac{n}{2}\)(a + l)
S20 = \(\frac{20}{2}\)(11 + (-65))
= 10(-54)
= -540
Therefore, the sum of first 20 terms of this A.P. is -540.