Answer:
Uncle Mitch explained to me that the Magna Carta was written by English nobles to curb the power of King John I.
Step-by-step explanation:
Whenever someone uses uncle, and there is a name, then it's capitalized. Magna Carta should be capitalized because it's a name. English also needs to be capitalized because it's a proper noun. Again, power shouldn't be capitalized because it isn't a proper noun, but John should be because it's part of a name.