Question 324311
Let x be one  number and y  be the other one.
You are told
{{{x+ y = 51}}}
{{{x = 2y + 3}}}
Use substition
{{{x+ y = 51}}}
{{{(2y+3) + y = 51}}}
{{{3y + 3 = 51}}}
{{{3y = 48}}}
{{{y = 16}}}
If y is 16, then x is {{{2y+3 = 2*16 + 3 = 35}}}
Check your answer. Does 16+35 = 51?