Question 1065175
The distributive property (or distributive law of multiplication) says this:
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            a*(b+c) = a*b + a*c
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The 'a' is "distributed" across all terms inside the parenthesis ('terms' are things separated by a + or - sign).

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Adding extra steps to show how 7(45) fits, note that 45 = 45 + 0:   7(45) = 7(45+0) = 7*45 + 7*0 = 315

The distributive property is important to know, as it is used virtually every time  when re-arranging algebraic equations, often in the process of solving them for unknown(s).