Try something like this
$esx=Get-VMHost$start= (Get-Date).AddMonths(-2)$metric="cpu.usage.average","mem.usage.average"
Get-Stat-Entity$esx-Start$start-Stat$metric|
Group-Object-Property {$_.Entity.Name} |Select@{N="VMHost";E={$_.Name}},
@{N="Avg CPU %";E={$_.Group|where {$_.MetricId-eq"cpu.usage.average"} | Measure-Object-PropertyValue-Average|Select-ExpandPropertyAverage}},
@{N="Avg Memory %";E={$_.Group|where {$_.MetricId-eq"mem.usage.average"} | Measure-Object-PropertyValue-Average|Select-ExpandPropertyAverage}}