that is the heavy cannon required research.Engineering Breakthrough Fuel Injection Engine Power Module Machinegun Hardened MG Bullets Cyborg Factory Factory Module Cobra Sensor Turret Guard Tower Sensor Tower Command Relay Post Research Module Composite Alloys Bug Composite Alloys MK2 Python Light Cannon HEAT Cannon Shells Medium Cannon Heat Cannon Shells Mk2,Mk3 Heavy Cannon
now as your doing t1 - build 5 research - myself personally starts with the mortar then he minirockets then heat cannon shells then apdsb machine gun bullets then huricane aa turret... and follow the progressions of those - as well as the upgrades to the power and factories. just make sure your oil / power levels are adequate. dont forget to do the research on the wall upgrades as well. best tip of all though - dont panic - ever. ;D