If 3,x, y,18 are in arithmetric progression , find the value of x and y

Given:
arithmetic progression
a₁ = 3
a₂ = x
a₃ = y
a₄ = 18

Solution:
The arithmetic progression has general formula to determine nth term
a₁ + d(n - 1) = an
a₁ represents first term, d represents difference, n represents number of terms, an represents nth term

First we should find out the value of d, by submitting a₄ to the formula
a₁ + d(n - 1) = an
a₁ + d(4 - 1) = a₄
3 + d(3) = 18
3 + 3d = 18
3d = 15
d = 5
The difference of the sequence is 5

Second, determine x and y
x = a₂
x = a₁ + d(2 - 1)
x = 3 + 5(1)
x = 8

y = a₃
y = a₁ + d(3 - 1)
y = 3 + 5(2)
y = 13

The value of x is 8, the value of y is 13