the arithmetic sequence members are

a, a+d, a+2d, a + 3d, ...

a is the first term

d is the "common difference" between terms

in our case we have

a = 124

a + d = 133

d = 133 - a

d = 133 - 124

d = 9

or we can write

a0 = 24

a1 = 24 + 9

a2 = 24 + 2*9

.

.

.

an = 24 +n*9

the sum of the first n members is calculated by the formula

Sn = a0 + a1 + ... + an-1 = (n/2)(2*a + (n - 1)d)

in our case a = 24, d = 9, n = 35 that is

S34 = (34/2)(2*24 + (34 - 1)*9)

S34 = 5865

the sum of the first 35 numbers in the arithmetic series is 5865.