Find the unknown number in each equation.

5a-3a= 18
a=

5a-3a= 18

2a = 18

Divide both sides of the equation by 2.

a = 9

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You can prove this by substituting 9 for a in the original equation.

5(9) - 3(9) = 18

45 - 27 = 18

Yep!! It's right.