Question 1178384
This seems to be like  you have points on a line  (10,40), (20,70), and (25,c).


slope {{{(7-4)/(2-1)=3}}}.


If trying point-slope equation, {{{y-40=3(x-10)}}}
{{{y=3x-30+40}}}
{{{y=3x+10}}}
but you want c=y, and x=25.


{{{c=3*25+10}}}
{{{highlight(highlight(c=85))}}}