You have to make a simultaneous equation.
For the first case, the equation is x+y= 11 or y=11-x and for the second is x*y=-60
When plot the y from the first in the second equation, giving us x(11-x)=-60 or -x^2+ 11x=-60 or even x^2-11x-60=0
And using complete the square, (x-5.5)^2 - 30.25-60=0 , simplifying into (x-5.5)^2 = 90.25 and x-5,5=sqrt(90.25)
x-5.5=9.5 and x=15 or x-5.5=-9.5 and x = -4.
Back to the equation x*y=-60 and making y the subject, being y=-60/x
we replace the two values of x, giving us y=-60/15=-4 or y=-60/-4= 15. Hence the possible answers are 15 and -4 or -4 and 15.
