Question 210627
Since "A and B are supplementary", this means {{{A+B=180}}}. So plug in {{{A=(3/8)x+50}}} and {{{B=x+31}}} into {{{A+B=180}}} to get {{{(3/8)x+50+x+31=180}}}. Now just solve for 'x' and use this value of 'x' to find A and B.