help with arithmetic Progression

Question

I need to answer this using an Ap equation. Given there are 32 students in Week 3 and 12 students in Week 13, find the number of students attending the lecture in Week 7.

Answer 1

a₃ = 32

a₁₃ = 12

 

a₁₃ = 32 + (10)d

12 = 32 + 10d

-10 = 10d

-1 = d

 

Can you determine a₁ and then a₇?