# How do you solve x^2+3x-70=0 by completing the square?

Jul 6, 2015

When the $x$-coefficient (in this case $3$) is odd, completing the square is not the best method.

#### Explanation:

Consider the sum-product-method instead: The task is to factor $- 70$ into two factors that add up to $3$.

After some trying you may find that $10 \mathmr{and} - 7$ will do the job.
Now you can factorise the left part of the equation:

$\to \left(x + 10\right) \cdot \left(x - 7\right) = 0$ so either:
$x + 10 = 0 \to x = - 10 \mathmr{and} x - 7 = 0 \to x = 7$