49x^2y^2- (7xy -c)^2= 49x^2y^2- (49x^2y^2-14xyc+c^2)= 49x^2y^2- 49x^2y^2+14xyc-c^2=14xyc-c^2=c(14xy-c)
49x^2y^2- (7xy -c)^2= 49x^2y^2- (49x^2y^2-14xyc+c^2)= 49x^2y^2- 49x^2y^2+14xyc-c^2=14xyc-c^2=c(14xy-c)