Answer
$79.45
Use the <em>simple interest formula</em>
A= P(1+rt)
A = 70(1+0.045×3)
And you solve, which gives you 79.45
Given

we have

Squaring both sides, we have

And finally

Note that, when we square both sides, we have to assume that

because we're assuming that this fraction equals a square root, which is positive.
So, if that fraction is positive you'll actually have roots: choose

and you'll have

Which is a valid solution. If, instead, the fraction is negative, you'll have extraneous roots: choose

and you'll have

Squaring both sides (and here's the mistake!!) you'd have

which is not a solution for the equation, if we plug it in we have

Which is clearly false
Subtract the 17190 from the 21840. you get 4650. 4650 divided by 21840 is 0.21. The percent decrease is 21%
I believe your answer is C. But I’m not completely sure!