When the graph intersects the y-axis, it means it's x-coordinate is 0, or t = 0.
Hence, f(t) = 6 × 2^0 = 6.
y = 6 at y-intercept.

Setting

, you have

. Then the integral becomes




Now,

in general. But since we want our substitution

to be invertible, we are tacitly assuming that we're working over a restricted domain. In particular, this means

, which implies that

, or equivalently that

. Over this domain,

, so

.
Long story short, this allows us to go from

to


Computing the remaining integral isn't difficult. Expand the numerator with the Pythagorean identity to get

Then integrate term-by-term to get


Now undo the substitution to get the antiderivative back in terms of

.

and using basic trigonometric properties (e.g. Pythagorean theorem) this reduces to
The perimeter of a triangle is found by adding all the three sides.
Let's call the third side "x"
So,
6+9+x = 23
15+x = 23
x = 8
The amount after 7 years is given by:

The answer is: $20,948.15