Find the outer perimeter of this figure

Naomi’s parents want to have 50,000, saved for her college education, if they invest 20000 today and earn 7% interest compound annually, about how long will it take them to save 50 thousand

Identify the initial amount a and the growth factor b in the exponential function. a(x)=680*4.3^x

Iknow its nothing to do with education, but what type of thing should i wrote here? ? : ) its for mcdonald's btw.