[tex]15.\\7x^4-5x^4\qquad\text{combine like terms}\\\\=(7-5)x^4=\boxed{2x^4}\\\\17.\\6b+7b-10\qquad\text{combine like terms}\\\\=(6+7)b-10=\boxed{13b-10}\\\\19.\\y+4+3(y+2)\qquad\text{use the distributive property}\ a(b+c)=ab+ac\\\\=y+4+(3)(y)+(3)(2)=1y+4+3y+6\qquad\text{combine like terms}\\\\=(1+3)y+(4+6)=\boxed{4y+10}\\\\21.\\3y^2+3(4y^2-2)\qquad\text{use the distributive property}\ a(b-c)=ab-ac\\\\=3y^2+(3)(4y^2)-(3)(2)=3y^2+12y^2-6\qquad\text{combine like terms}\\\\=(3+12)y^2-6=\boxed{15y^2-6}[/tex]
