I think the answer is x=10
I did not catch well, but i think you already got your answers. Let me explain. For the first part you got your notation right: for 0.0045 the correct scientific notation is 4.5 x 10^-3. Now the second part, the answer would be the first one, the notation that you are giving is the one who corresponds to <span>4.5 x 10^-2, the first option you have. Hope it helps</span>
Let us try and solve it analytically. We have that the side=x+3 together with the short side=s and the diagonal=x+4 satisfy the Pythagorean Theorem. Then we have that

. This yields

which yields s^2=2x+7, hence a) is the correct answer.