What geometric figure the riddle represent​

Which is an equation for the nth terms of the sequence 12,15,18,21

What values of x satisfy the inequality x^2-9x< 0? how is your answer shown on the graph?

William invested $5000 in an account that earns 3.8% interest, compounded annually. the formula for compound interest is a(t) = p(1 + i)t. how much did william have in the account after 6 years? (apex)