Tesla Motors Inc. on Monday said it had reached a deal to buy SolarCity Corp. for less than the price it originally proposed, as Elon Musk takes the next step forward with his plan to combine his electric-car and solar-energy companies.

The all-stock deal values SolarCity at about $2.6 billion, with SolarCity stockholders receiving 0.11 share of Tesla for each share of SolarCity, valuing them at $25.83 apiece, according to Friday’s closing prices.

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