Question 959129

to be a  perfect square trinomial should be {{{y=(x+c)^2}}}

recall the rule: {{{(x+c)^2=x^2+2*1*c+c^2}}}

so, you have {{{y=x^2+15x+c}}} and {{{2*1*c=2c=15}}}

then  the value of {{{c}}} is   {{{15/2=7.5}}} 

{{{y=(x+7.5)^2 }}} 

so, {{{ c = 7.5}}}