The congruence 7x≡21(mod14) is the same as
x≡3(mod2)
The congruence 2x+13≡16(mod9) is the same as 2x≡3(mod9) and therefore to
x≡15(mod9)
because 2⋅5≡1(mod9)
The congruence −2x+1≡x(mod25) is the same as 3x≡1(mod25) and therefore to
x≡17(mod25)
because 3⋅17≡1(mod25)
Thus you have
x≡1(mod2)
x≡6(mod9)
x≡17(mod25)
The first one tells you that x=2a+1 for some integer a . Thus we need
2a+1≡6(mod9)
so 2a≡5(mod9) and therefore
a≡25≡7(mod9)
Hence a=9b+7 for some integer b. Now we have
x=2a+1=2(9b+7)+1=18b+15
and so we need
18b+15≡17(mod25)
so 9b≡1(mod25)
Multiplying by 3 we have 27b≡3(mod25) , so 2b≡28(mod25) and finally
b≡14(mod25)
Thus b=25c+14 and
x=18(25c+14)+15=450c+267
In order to get a four digit number we need
450c+267>999
hence c>122/75 , so c=2 is the least integer and we get
450⋅2+267=1167 - is the smallest 4-digit integer.