two angles are called supplementary when the sum of them is 90 degrees (right angle)
in this case we have. the supplementary angle x and angle y
let x=5y+6 and x+y = 90 then. x=90-y substituting this value of x to the first equation, weget:
90-y=5y+6 90-6=5y+y 84=6y and y= 84/6 = 14 so x= 90-14 = 76 which is 5*14+6