Qn: Given that 3, b, c and 18 are an Arithmetic Progression. Find the value of b and c.

Answer 1

an=a1+(n-1)*d

In your case:
a1=3
n=4
a4=18

a4=a1+(4-1)*d

18=3+3*d
18-3=3d
3d=15 Diwide booth sides with 3

d=5

an=a1+(n-1)*d

a1=3+0*5=3

a2=3+5*1=3+5=8

a3=3+2*5=3+10=13

a4=3+3*5=3+15=18

Progresion:
3,8,13,15

In google type:
arithmetic progression

When you see list of results click on some site.

wikipedia is good choice