Instead of substituting in and then solving, try the other way around -- solve then substitute. You'll end up with the same result either way, but algebra is easier with variable names than with numeric values. And errors are easier to find and correct.
Typesetting instructions can be found
here.
Let's type set that starting equation using TeX. Putting a double dollar sign in front and a double dollar sign behind you get to render a mathematical equation centered on the line.
This ($$E_k=\frac 1 2 mv^2$$) becomes this:$$E_k = \frac 1 2 mv^2$$One can also do it with doubled hash marks to get inline equations. This (##E_k=\frac 1 2 mv^2##) becomes ##E_k=\frac 1 2 mv^2##
Let's solve the above equation for ##v##. Multiply both sides by 2 and divide both sides by m. That gives us $$\frac{2 E_k}{m} = v^2$$ Now take the square root of both sides and swap right and left yielding $$v = \sqrt{ \frac{2 E_k}{m} }$$ But of course, you already knew that.
Now you can sutstitute in 3.1 x 10
6 J for ##E_k## and 750 kg for m yielding $$v=\sqrt{\frac{2 \times 3.1 \times 10^6 \text{J}} {750 \text{kg}}}$$
[Cheating to display back to back dollar signs and hash marks is tricky. I pulled down the little paint droplet icon to pick a color for the second dollar sign, making it explicitly black. Enclosed in a color tag that way, the character was no longer able to activate the equation rendering engine]
). If your system is just the car, we have external work and this equation won't apply.
