This reaction is an E2 elimination reaction, it involves a single-step mechanism. In the single step mechanism, the required geometry is an anti-peri-planar geometry. In other words, the required configuration is a configuration for the leaving group and the H that would be removed. The leaving group and the hydrogen must now be of an opposite configuration when the base attacks the hydrogen.
Hence the stereo chemistry of the product (E)-(2-methylbut-1-en-1-yl)benzene shown below.