Question 163534
First, name your variables. 
How about A, B, and C?
Then turn the words into mathematical equations.
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"The sum of three numbers is 57."
1.{{{A+B+C=57}}}
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"The second is 3 more than the first."
2.{{{B=A+3}}}
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"The third is 6 more than the first."
3.{{{C=A+6}}}
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Now substitute eq. 2 and eq. 3 into eq. 1 and solve for A.
 1.{{{A+B+C=57}}}
{{{A+(A+3)+(A+6)=57}}}
{{{3A+9=57}}}
{{{3A=48}}}
{{{highlight(A=16)}}}
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Now that you have A you can find B and C using eq. 2 and eq. 3,
2.{{{B=A+3}}}
{{{B=16+3}}}
{{{highlight(B=19)}}}
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3.{{{C=A+6}}}
{{{C=16+6}}}
{{{highlight(C=22)}}}
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Check your answer.
{{{A+B+C=57}}}
{{{16+19+22=57}}}
{{{57=57}}}
True statement.
Good answers.