Answer:

10

Step-by-step explanation:

3,9,15,21,...,57

d/common difference= 9-3= 6

a1/Ist term= 3

an/last term= 57

n/nth term= ?

Formula

an=a1+(n-1)d

Solution

Substituting> 57=3+(n-1)6

Solve> 57=3+6n-6

57= 3-6+6n

57= -3+6n

57+3=6n

Divide> 60/6 = 6n/6

10=n