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Weighted averages

Point A\pink{A} is at (10,9)(-10, -9) and point B\blue{B} is at (5,4)(5, -4).
If point M=(1t)A+tB=(4,7)\purple{M} = (1 - t)\pink{A} + t\blue{B} = (-4, -7), what is the value of tt?
You can use the interactive diagram below to help you :