F4 math problem~!!

(a) The height x (cm) of afather is more than 3 times the height y (cm) of his baby son by 10 cm
=> x = 3y + 10
If the height of the father were 36 cm more,while the height of his baby were 36 cm less, then the father's height will be equal to the square of that of his baby son
=> x+36 = (y-36)²
So x = 3y + 10 and x = y² - 72y + 1260

(b) Put x = 3y + 10 into x = y² - 72y + 1260
3y+10 = y² - 72y + 1260
y² - 75y + 1250 = 0
(y-50)(y-25) = 0
y = 50 or y = 25
If y = 50, x = 3y + 10 = 160
If y = 25, x = 85
Since the height of the baby is more than 36cm from the second condition, the solution pair y = 25 and x = 85 is rejected.
So x = 160, y = 50.
The heights of the father and his son are 160cm and 50cm, respectively.

x=3y+10....(1)
x+36=(y-36)^2....(2)
(1)代入(2)....(3y+10)+36=(y-36)^2
　　　　　 3y+46=y^2-2(y)(36)+36^2
3y+46=y^2-72y+1296
y^2-72y+1296-3y+46=0
y^2-75y+1342=0
y=29.48434或45.51560