Question 935824
first term is {{{5 + x}}}, 
second term is {{{8}}}, and 
third term is {{{1 + 2x}}}

you know that between each term is same difference:

{{{8-(5 + x)}}} is same as {{{(1 + 2x)-8}}}

make it equal and solve for {{{x}}}

{{{8-(5 + x)=(1 + 2x)-8}}}

{{{8-5 -x=1 + 2x-8}}}

{{{3 -x=2x-7}}}

{{{3 +7=2x+x}}}

{{{10=3x}}}

{{{10/3=x}}}

{{{3+(1/3)=x}}}

first term is {{{5 + x=5+3+(1/3)=8+(1/3)}}}, =>{{{8}}}{{{(1/3)}}}
second term is {{{8}}}, and 
third term is {{{1 + 2x=1+2(3+(1/3))=1+6+2/3)=7+(2/3)}}} =>{{{7}}}{{{(2/3)}}}