Solve for a
a=21
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5a+15=4\left(a+9\right)
Use the distributive property to multiply 5 by a+3.
5a+15=4a+36
Use the distributive property to multiply 4 by a+9.
5a+15-4a=36
Subtract 4a from both sides.
a+15=36
Combine 5a and -4a to get a.
a=36-15
Subtract 15 from both sides.
a=21
Subtract 15 from 36 to get 21.
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