763+[3·[239+3a/5]-854]·2=867
⇒[3·[239+3a/5]-854]·2=867-763
⇒[3·[239+3a/5]-854]·2=104
⇒3·[239+3a/5]-854=104/2
⇒3·[239+3a/5]-854=52
⇒3·[239+3a/5]=854+52
⇒3·[239+3a/5]=906
⇒239+3a/5=906/3
⇒239+3a/5=302
⇒3a/5=302-239
⇒3a/5=63
⇒3a=5·63
⇒3a=315
⇒a=105[cm-latura triunghiului echilateral];
⇒P Δechilateral=3a
⇒P Δechilateral=3·105cm
⇒P Δechilateral=315cm;
⇒l patrat=P Δechilateral
⇒l patrat=315cm
⇒P patrat=4l
⇒P patrat=4·315cm
⇒P patrat=1260cm[perimetrul patratului];