Question 167929
If that's a right triangle, then {{{A+B+C=180^o}}} right?
Since {{{A=21^o}}} in that right triangle {{{C=90^o}}}, then
{{{21+B+90=180}}}
{{{B=180-90-21}}}
{{{B=69^o}}}
Let's see the  triangle below:
{{{drawing(300,300,-5,12,-5,6,graph(300,300,-5,12,-5,6),blue(line(1,1,10,1)),green(line(10,1,10,4)),red(line(10,4,1,1)),green(circle(10,1,.20)),locate(0.7,0.9,A)locate(10.2,4,B),locate(10.2,1,C))}}} ---> RED Line {{{AB}}}=unknown?; Blue Line {{{AC}}}=13"; GREEN Line {{{BC}}}=5"

Now, if {{{BC=5}}}, by trigo functions ----> {{{sin(alpha)=opp/hyp=BC/AB}}}
{{{sin21^o=5/AB}}}
{{{AB=5/sin21^o=highlight(13.952inches=AB)}}}
Check: Remember in a right triangle:
{{{hyp^2=opp^2+adj^2}}} ---> {{{AB^2=BC^2+AC^2}}}
{{{13.952^2=5^2+13^2}}}
{{{194.658=25+169}}}
{{{194.658>=194}}}, close enough, round off.
Thank you,
Jojo