If triangles PQR and STU are similar then PQ corresponds to ST and PR corresponds to SU. Therefore, PQ/ST=PR/SU Considering that, PQ= 7-x, ST= 13-x, PR= x²+5 and SU= x² +20 therefore, (7-x)/(13-x)= (x²+5)/(x²+20) cross multiplying, 7x² +140-x³+20x =13x²+65-x³-5x combining the like terms, 6x² +15x -75=0 solving for x, x = 5/2 or -5
