Resposta:

a) 5x=15

    x=15/5=3

b) 2x=42

x= 42/2= 21

c) 7x-2x=210-90

   5x= 120

   x= 120/5= 24

d) tá faltando coisa ai não tá não?

e) x= 400/4= 100

Explicação passo a passo: nem sei

[tex]\text{\Large\\ \begin{array}{l}\mathbf{a) \ 2x+3x=15}\\5x=15\\x=\frac{15}{5} \ =\ \boxed{3}\\\\\mathbf{b) \ 5x\ -\ 3x\ =\ 42}\\2x=42\\x=\frac{42}{2} \ =\ \boxed{21}\end{array}}[/tex]

[tex]\text{\Large\\ \begin{array}{l}\mathbf{c) 7x\ 90\ =\ 2x\ +\ 210}\\7x-2x=210-90\\5x=120\\x=\frac{120}{5} \ =\ \boxed{24}\\\\\mathbf{d) \ 3x\ =\ 150\ -}\\x=\frac{150}{3} \ =\ \boxed{5}\\\\\mathbf{e) \ 4x\ =\ 400}\\x=\frac{400}{4} \ =\ \boxed{100}\end{array}}[/tex]