Question 153390
If two angles are complementary, their sum is 90 degrees.
So 
{{{A+B=90}}}
Substituting A=5x+8 and B=x+4 into the equation above, we have
{{{5x+8+x+4=90}}}
{{{6x+12=90}}}
{{{6x=78}}}
{{{x=78/6}}}
{{{x=13}}}
Substituting x = 13 into A=5x+8 and B=x+4, we have
{{{A=5x+8=5*13+8=73}}}
{{{B=x+4=13+4=17}}}