Please help I don't understand?

I have a maths question that I have been working on for ages and I just dont understand how to do it here is the question : here is a rectangular tank it has four sides a base but no lid the lengths are 2.5m in height 4m across and 2m down the smaller side could you work out the surface area please but remember there is no lid could you possible show me the workings out aswell thankyou so much xx