Why doesn't this code work?

0=\left{ \begin{array}{lr} 2u^2-7u+7 & \text{if } x \ge 1 \\ 2u^2+7u+7 & \text{if } x<1 \end{array} \right.

Which gives:

$\displaystyle 0=\left{ \begin{array}{lr} 2u^2-7u+7 & \text{if } x \ge 1 \\ 2u^2+7u+7 & \text{if } x<1 \end{array} \right. $