5 ONERRORGOTO500
10 MODE8:DIM:ERASEa:DIMa(5):a$="x1y1x2y2ab":c$=CHR$(5)
20 CLS:PRINT"y=ax+b - (x1,y1),(x2,y2)";
30 FORi=0 TO3
40 LOCATE0,1:PRINTc$;MID$(a$,1+2*i,2);"=";a(i);"?";:INPUT@42;a(i):LOCATE0,0
50 NEXT:LOCATE0,1:PRINTc$;
70 IFa(0)=a(2) THENIFa(1)=a(3) THEN100 ELSEPRINTc$;"x =";a(0);:w$=INPUT$(1,@):GOTO30
72 IFa(1)=a(3) THENPRINTc$;"y =";a(1);:w$=INPUT$(1,@):GOTO30
75 a(4)=(a(3)-a(1))/(a(2)-a(0)):a(5)=a(1)-a(4)*a(0)
80 FORi=4 TO5:PRINTc$;MID$(a$,i+5,1);" =";a(i);
82 w=ASC(INPUT$(1,@)):IFw=13 THEN90
84 IFw=30 ANDi>4 THENi=i-2:GOTO90
85 IFw=31 ANDi<5 THEN90
88 GOTO82
90 NEXT:GOTO30
100 LOCATE0,1:PRINTc$;"not found";:w$=INPUT$(1,@):GOTO20
500 IFERR=1 THENCLS:ONERRORGOTO0
510 IFERL=40 THENRESUME40
520 RESUME100