Apply Pythagorean theorem first
[tex]\\ \rm\rightarrowtail b^2=11^2-10^2[/tex]
[tex]\\ \rm\rightarrowtail b^2=121-100[/tex]
[tex]\\ \rm\rightarrowtail b^2=21[/tex]
[tex]\\ \rm\rightarrowtail b=4.2\approx 4units[/tex]
Now
[tex]\\ \rm\rightarrowtail \dfrac{1}{2}(x)(10)=75[/tex]
[tex]\\ \rm\rightarrowtail 5x=75[/tex]
[tex]\\ \rm\rightarrowtail x=15units[/tex]
