You have done very well in typing the EXACT answer for the exponent with base e
It is indeed ln(0.9)
For the last part, they are asking you to round the answer in decimal form and rounded to THREE decimals.
So as we calculate the total expression for y with the calculator, we get:
y = 2.6 * e^(ln(0.9))
y = 2.6 * e^(-0.105 x)
where we have rounded the ln(0.9) that the calculator shows as: -0.1053605... to THREE decimals as requested.
Please make sure you enter the NEGATIVE sign in the exponent, which indicates an exponential DECAY.
When you use the calculator to evaluate the natural log of 0.9, you obtain:
ln(0.9) = -0.10536051565...
SImply by typing the expression on the left of the equal sign.
That ln(0.9) is the value that should be multiplying the "x" in the exponent.
Notice that the value is NEGATIVE (indicating an exponential DECAY) (the shape of the function is going to be reducing - going down - as you move from left to right.
Now, the next step is to write the exponential form with the exponent that comes from evaluating the logarithm. Your teacher wants you to express it with just THREE decimal values, instead of the many decimals that the calculator shows. So, we make sure that we round the value to the THIRD decimal, and not any other decimal value that follows. Then we stop at : -0.105