№1.
Ответ: 
№2.
\left \{ {{5a-3b=11} \atop {b=1-3a}} \right. =>\left \{ {{5a-3(1-3a)=11} \atop {b=1-3a}} \right. \\5a-3(1-3a)=11\\5a-3+9a=11\\5a+9a=11+3\\14a=14\\a=1\\\left \{ {{a=1} \atop {b=1-3a}} \right. => \left \{ {{a=1} \atop {b=-2}} \right." alt="\left \{ {{5a-3b=11} \atop {3a+b=1}} \right. =>\left \{ {{5a-3b=11} \atop {b=1-3a}} \right. =>\left \{ {{5a-3(1-3a)=11} \atop {b=1-3a}} \right. \\5a-3(1-3a)=11\\5a-3+9a=11\\5a+9a=11+3\\14a=14\\a=1\\\left \{ {{a=1} \atop {b=1-3a}} \right. => \left \{ {{a=1} \atop {b=-2}} \right." align="absmiddle" class="latex-formula">
Ответ: a=1; b=-2
№3.
а) (2a-3b)(5a+b)-10(a+b)²=(10a²+2ab-15ab-3b²)-10(a²+2ab+b²)=(10a²-13ab-3b²)-(10a²+20ab+10b²)=10a²-13ab-3b²-10a²-20ab-10b²=-33ab-13b²= -b(33a+13b)
б) 
№4.
a) x+xy-xz=x(1+y-z
б) x²-9y²+x-3y=(x-3y)(x+3y)+(x-3y)=(x-3y)((x+3y)+1)=(x-3y)(x-3y+1)
№5.
x - V пешехода
y - V велосипедиста
\left \{ {{5x=2(x+6)} \atop {y=x+6}} \right. \\5x=2(x+6)\\5x=2x+12\\5x-2x=12\\3x=12\\x=4\\\left \{ {{x=4} \atop {y=x+6}} \right. => \left \{ {{x=4} \atop {y=10}} \right." alt="\left \{ {{5x=2y} \atop {y=x+6}} \right. => \left \{ {{5x=2(x+6)} \atop {y=x+6}} \right. \\5x=2(x+6)\\5x=2x+12\\5x-2x=12\\3x=12\\x=4\\\left \{ {{x=4} \atop {y=x+6}} \right. => \left \{ {{x=4} \atop {y=10}} \right." align="absmiddle" class="latex-formula">
Ответ: скорость пешехода 4 км/ч; велосипедиста - 10 км/ч