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rth term and general term need help with formula please

My problem:

Find the coefficient of the term containing $\displaystyle x^3$ in the expansion of $\displaystyle (1 - \sqrt{x})^8$

I have to find the general term and compare it with the required term.

I need to apply the rth formula in the expansion of $\displaystyle (a+b)^n$ to find the general term.

I figured out that n = 8 and a = 1.

**What I don't understand is how the exponent for b which equals x = $\displaystyle \frac{1}{2}$**

**TIA...**

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