请问高手如何利用vb去求解分段函数y=4*X+2 [0,4] X*X+2 [4,8]最大值呢?

y=x+2 [0,4]
y=x*x-6*x+12 [4,8],
分别输入0、4、8
private sub command1_click()
x1=val(text1.text)
x2=val(text2.text)
x3=val(text3.text)
t=x1+2
s=x2*x2-6*x2+12
for i = x1 to x2 step 0.01
y=i+2
if y>t then t=y
next i
for i = x2 to x3 step 0.01
y=i*i-6*i+12
if y>s then s=y
next i
msgbox("区间[" & x1 & "," & x2 &"]" & "最大值为" & t & chr(13) & "区间[" & x2 & "," & x3 &"]" & "最大值为" &s )
end su