How to increase credit history?

I was recently denied a $3k loan at my bank even with a 750 score and 55k salary. I wanted to purchase a tractor and didn't want to pay cash to build more credit. They said my work history and credit history were both too short. In 2 years, I will like to get a mortgage for a home. If I got denied $3k, how can I work towards having enough history to qualify for a mortgage? I'm thinking about buying the tractor with in store credit, which I will probably use later on anyway to buy more appliances, is this ok? Is there other ways to build credit besides my already credit card?

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